The date of Easter and its lunisolar computation Computus is of central importance to many Christian churches. It is historically linked to the Jewish holiday of Pessach, Passa or Passover, which begins the evening before the 15th day of the lunar month of Nisan. The First Council of Nicaea settled on a set of rules in 0325, because there were several alternative customs. After a while, almost every sect was celebrating Easter on the same day, the first Sunday after the first nominal (ecclesiastical) full moon after the nominal Northern spring equinox in the Julian calendar (or equivalently the Coptic calendar or Ethiopian calendar). The Gregorian calendar reform caused a schism in 1582, because it hasn’t been adopted by all churches and those that did didn’t do it all at the same time. Some churches even stayed with the old method of determining the date of Easter while adopting the Gregorian calendar for all other purposes.
Wikipedia has more, well-sourced facts on the details and history:
As the last of these articles illustrates, there are still proposals floated to reform the Computus once again. The goals vary, but usually proponents want to make the date more universal, predictable, static or accurate.
All efforts to agree on an even more astronomically “correct” date than achieved by the early-modern Gregorian algorithm already – the last one from 1997 – have failed, unless one counts the Revised Julian calendar, introduced in 1923, with a 900-year leap cycle that is aligned with the 400-year Gregorian leap cycle from 1600 through 2800. Some Orthodox leaders of Eastern churches are not ashamed to admit that for them to consider to finally abandon the Julian calendar in order to achieve a common date of celebrations for all Christianity again, they expect Western churches, the Roman Catholic one in particular, to also have to make some serious concessions. The Aleppo Easter dating method would have differed from the Gregorian method at most 1 in 19 years for the foreseeable future, but almost every year for the Orthodox rites. It may therefore make sense to put forth a more ambitious reform proposal that stays true to the Nicaea rules, though slightly bent perhaps.
DOY | 081 | 082 | 083 | 084 | 085 | 086 | 087 | 088 | 089 | 090 | 091 | 092 | 093 | 094 | 095 | 096 | 097 | 098 | 099 | 100 | 101 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 | 116 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Jan01 | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sun | Sat | Fri |
DL | D | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC |
SOY | 12th | 13th | 14th | 15th | 16th | 17th | ||||||||||||||||||||||||||||||
ISO | W12-7 = M03-W4-7 = Q1-3-W3-7 | W13-7 = M04-W1-7 = Q1-3-W4-7 | W14-7 = M04-14 = Q2-1-W1-7 (Sym) | W15-7 = M04-W3-7 = Q2-14 | W16-7 = M04-W4-7 = Q2-1-W3-7 | W17-7 | ||||||||||||||||||||||||||||||
Common | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 |
Leap | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
Lent | 1 pre-Lenten Sunday | 2 pre-Lenten Sundays | 3 pre-Lenten Sundays: Su = 100±3 | 4 pre-Lenten Sundays | ||||||||||||||||||||||||||||||||
DOYs | PC | TWC | Su ≻ 099 | |||||||||||||||||||||||||||||||||
Su ≺ 100 | Su ≻ 100 |
DOM | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 01 | 02 | 03 | 04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
DOMar | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | ||||||||||
Mar01 | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon | Sun | Sat | Fri | Thu | Wed | Tue | Mon |
DL | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC | ED | FE | GF | AG | BA | CB | DC |
SOY | 12th | 13th | 14th | 15th | 16th | 17th | |||||||||||||||||||||||||||||
12th | 13th | 14th | 15th | 16th | 17th | ||||||||||||||||||||||||||||||
DOY | 081 | 082 | 083 | 084 | 085 | 086 | 087 | 088 | 089 | 090 | 091 | 092 | 093 | 094 | 095 | 096 | 097 | 098 | 099 | 100 | 101 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 |
082 | 083 | 084 | 085 | 086 | 087 | 088 | 089 | 090 | 091 | 092 | 093 | 094 | 095 | 096 | 097 | 098 | 099 | 100 | 101 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 | 116 | |
ISO | W12-7 | W13-7 | W14-7 | W15-7 | W16-7 | ||||||||||||||||||||||||||||||
W12-7 | W13-7 | W14-7 | W15-7 | W16-7 | W17-7 | ||||||||||||||||||||||||||||||
Equinox | Su ≻ Mar21 | 2Su ≻ Mar21 | 3Su ≻ Mar21 | 4Su ≻ Mar21 | 5Su ≻ Mar21 | ||||||||||||||||||||||||||||||
Historic | Su ≈ Apr09 (30 AD) | ||||||||||||||||||||||||||||||||||
Su ≈ Apr10 (Pepuzite = Montanist) | |||||||||||||||||||||||||||||||||||
Month | Su ≺ Apr | Su ≻ Mar | 2Su ≻ Mar (Coptic etc.) | 3Su ≻ Mar | |||||||||||||||||||||||||||||||
Lady Day | Su ≻ Mar25 | 2Su ≻ Mar25 = Su ≈ Apr05 (33 AD) | 3Su ≻ Mar25 = Su ≻ 2Sa ≻ Mar (Anglican) | 4Su ≻ Mar25 | |||||||||||||||||||||||||||||||
Christmas | 38Su ≺ Dec25 | 37Su ≺ Dec25 | 36Su ≺ Dec25 | 35Su ≺ Dec25 | |||||||||||||||||||||||||||||||
Week | 2Su ≻ Mar20 | Su ≻ Th ≻ Mar (-04-W1-7) = 3Su ≻ Mar20 | Su ≻ 2Th ≻ Mar (-04-W2-7) = 4Su ≻ Mar20 | Su ≻ 3Th ≻ Mar (-04-W3-7) = 4Su ≻ Mar20 |
Requirements and affordances[]
- The earliest possible date should not be earlier than the earliest Julian Easter in the 21st century, 4 April.
- The latest possible date should not be later than the latest Gregorian Easter, 25 April.
- The 14th or 15th day of some account may feel nice, like the Ides and Passover. With full-week months or quarters and Sunday at the …
- … end (ISO), the second Sunday of the fourth month or the second quarter is the 14th day.
- … beginning, the third Sunday of the fourth month or the second quarter is the 15th day.
- If fixed, a day-month date reminiscent of a plausible historic date would be welcome.
- 0030-04-09 and 0033-04-05 have been suggested as plausible Sundays after the crucifixion of a historic Jesus of Nazareth.
- Since the Gregorian reform skipped dates to match the proleptic calendar of the time of the First Council of Nicaea in 0325, the leap days in 0100, 0200 and 0300 might need accounting for.
- Between the traditional end of Christmastide (with Candlemas on 2 February) and the begin of Lent (on Ash Wednesday), there should be no more than 3 Sundays (which are known as Septuagesimae or Circumdederunt, Sexagesimae or Exsurge, Quinquagesimae or Estomihi), where there are currently up to five. Ideally, there would be either always exactly three or always none.
- The former ideal restricts the range of Ash Wednesday to 20–26 February, hence Easter Sunday to 7–13 April in common and to 6–12 April in leap years, i.e. the closest Sunday to ordinal day 100.
- The latter ideal conflicts with the Earliest Date restriction above.
- To match the ember days in mid-September (after the Exaltation of the Holy Cross) and mid-December (after the Feast of Saint Lucie), Ash Wednesday should be in mid-March and Pentecost should be in mid-June, while they need to stay 96 days apart. This would afford a late or very late Easter date.
Week Name | Offset | 12.1% | 23.5% | 23.2% | 23.3% | 17.6% | 0.3% | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
−9 | W03 | W04 | W05 | W06 | W07 | W08 | Circumdederunt | |||||||
−8 | W04 | W05 | W06 | W07 | W08 | W09 | Exsurge | |||||||
Fat | −7 | W05 | W06 | W07 | W08 | W09 | W10 | Fat | Shrove | |||||
Carnival | −6 | W06 | W07 | W08 | W09 | W10 | W11 | Rose | Pancake | Ash | Invocabit | |||
Spring ember | −5 | W07 | W08 | W09 | W10 | W11 | W12 | Reminiscere | ||||||
−4 | W08 | W09 | W10 | W11 | W12 | W13 | Oculi | |||||||
−3 | W09 | W10 | W11 | W12 | W13 | W14 | Laetare | |||||||
−2 | W10 | W11 | W12 | W13 | W14 | W15 | Passion | |||||||
Passa | −1 | W11 | W12 | W13 | W14 | W15 | W16 | Lazarus | Palm | |||||
Holy | ±0 | W12 | W13 | W14 | W15 | W16 | W17 | Holy | Holy | Holy | Maundy | Good | Holy | Easter |
Easter | +1 | W13 | W14 | W15 | W16 | W17 | W18 | Easter | Divine Mercy | |||||
+2 | W14 | W15 | W16 | W17 | W18 | W19 | Misericordia | |||||||
+3 | W15 | W16 | W17 | W18 | W19 | W20 | Jubilate | |||||||
+4 | W16 | W17 | W18 | W19 | W20 | W21 | Cantate | |||||||
+5 | W17 | W18 | W19 | W20 | W21 | W22 | Vocem jucunditatis | |||||||
Ascension | +6 | W18 | W19 | W20 | W21 | W22 | W23 | Ascension | Exaudi | |||||
Whit | +7 | W19 | W20 | W21 | W22 | W23 | W24 | Whitsun | ||||||
Pentecost | +8 | W20 | W21 | W22 | W23 | W24 | W25 | Pentecost | Trinity | |||||
Corpus | +9 | W21 | W22 | W23 | W24 | W25 | W26 | Corpus Christi | ||||||
Heart | +10 | W22 | W23 | W24 | W25 | W26 | W27 | Sacred Heart | Immaculate Heart |
Possible rules[]
Mnemonic | Notes |
---|---|
Western | 3rd Sunday of the paschal month in the Gregorian Lunar Calendar |
Su ≻ Su ≻ Su ≻ Mar = 3Su ≻ Mar |
3rd April Sunday |
Su ≻ Su ≻ Su ≻ Su ≻ Mar22 = 4Su ≻ Mar22 |
ecclesiastical Northward equinox fixed to March 22, which is guaranteed to be later than the astronomical one |
Su ≻ Su ≻ Su ≻ Su ≻ Mar21 = 4Su ≻ Mar21 |
ecclesiastical Northward equinox fixed to March 21 |
W15-7 | April 14 in new Hanke-Henry / Sym454/010; once proposed by John R “Merlyn” Stockton |
Su ≻ Su ≻ Su ≻ Su ≻ Mar20 = 4Su ≻ Mar20 |
ecclesiastical Northward equinox fixed to March 20 |
Su ≻ 100 | ordinal day 100 is April 10 in common or 09 in leap years |
36Su ≺ Dec25 | Christmas is on W51 Sunday in CB and B years, on W51 Saturday in C years and in W52 otherwise; so the Sunday before Christmas is usually in W51, rarely in W50; Lady Day Mar25 is in W12 in B, CB and C years, but also in G, AG, A and BA |
Su ≻ 099 = 16Su ≻ Dec25 |
ordinal day 099 is April 09 in common or 08 in leap years; 16th Sunday after Christmas |
Su ≻ Sa ≻ Sa ≻ Mar = Su ≻ 2Sa ≻ Mar |
British/Anglican proposal since at least 1920s |
15Su ≻ Dec | 3rd Sunday of the 4th 4-week month in some 13-month leap-week calendars |
World Calendar | WC April 08 = IFC April 15 = day 099 = April 09 in common or 08 in leap years; only a Sunday in A/AG years due to off-week dates; leap day occurs after June in the World Calendar and at the end of the year in the IFC, hence after all Easter-related holidays in both cases |
Su ≻ Su ≻ Mar = 2Su ≻ Mar |
2nd April Sunday, suggested e.g. by Coptic Pope Tawadros in 2015 |
Positivist Calendar | Archimedes 14 = day 098 = April 08 in common or 07 in leap years; only a Sunday in G/GF years due to off-week dates; leap day in the Positivist Calendar occurs at the end of the year, hence after all Easter-related holidays |
Pepuzites | closest to April 10, originally in the Julian calendar |
Su ≻ Su ≻ Su ≻ Mar22 = 3Su ≻ Mar22 |
ecclesiastical Northward equinox fixed to March 22, which is guaranteed to be later than the astronomical one |
Su ≻ Su ≻ Su ≻ Mar21 = 3Su ≻ Mar21 |
ecclesiastical Northward equinox fixed to March 21 |
W14-7 | April 7 in new Hanke-Henry / Sym454/010 with ISO 8601 leap week rules; 14th day of 4th 4-week month in ISO-conformant 13-month leap-week calendars |
Su ≻ Th ≻ Mar = Su ≻ Su ≻ Su ≻ Mar20 = 3Su ≻ Mar20 |
Sunday of the first ISO week in April "ccyy-04-W1-7"; ecclesiastical Northward equinox fixed to March 20 |
Su ≻ Su ≻ Mar25 = 2Su ≻ Mar25 = Su ≻ Sa ≻ Mar |
Palm/Passion Sunday as soon as possible after Annunciation of the Lord; Sunday after first April Saturday |
14Su ≻ Dec | after 1st 13-week quarter, ending with April 01 in common years and March 31 in leap years |
Su ≻ Mar | 1st April Sunday |
W13-7 | March 30 in new Hanke-Henry / Sym454/010 with ISO 8601 leap week rules; 7th day of 4th 4-week month in ISO-conformant 13-month leap-week calendars |
Rule | ISO week | Sunday of the year | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Mnemonic | after | before | ≤13 | 14 | 15 | ≥16 | ≤13 | 14 | 15 | 16 | ≥17 |
Western | 35,6% | 23,3% | 23,5% | 17,9% | 25,8% | 23,3% | 23,3% | 22,8% | 4,7% | ||
Eastern | 0,6% | 17,5% | 23,1% | 58,8% | 27,8% | 23,9% | 22,7% | 21,2% | 4,3% | ||
3Su ≻ Mar | April 14 | April 22 | 53¾% | 46¼% | 10¾% | 89¼% | |||||
4Su ≻ Mar22 | April 12 | April 20 | 82¼% | 17¾% | 39¼% | 60¾% | |||||
4Su ≻ Mar21 | April 11 | April 19 | 96¾% | 3¼% | 53¾% | 46¼% | |||||
W15-7 | 101 | 109 | 100% | 57 % | 43 % | ||||||
4Su ≻ Mar20 | April 10 | April 18 | 10¾% | 89¼% | 67¾% | 32¼% | |||||
Su ≻ 100 | 100 | 108 | 14½% | 85½% | 71½% | 28½% | |||||
36Su ≺ Dec25 | April 09 | April 17 | 25¼% | 74¾% | 82¼% | 17¾% | |||||
Su ≻ 099 | 099 | 107 | 28½% | 71½% | 85½% | 14½% | |||||
Su ≻ 2Sa ≻ Mar | April 08 | April 16 | 39¼% | 60¾% | 96¼% | 3¾% | |||||
15Su ≻ Dec | 098 | 106 | 43 % | 57 % | 100% | ||||||
World Calendar | 098 | 100 | 43 % | 57 % | 14½% | ||||||
2Su ≻ Mar | April 07 | April 15 | 53¾% | 46¼% | 10¾% | 89¼% | |||||
Positivist Calendar | 097 | 099 | 57 % | 43 % | 14 % | ||||||
Pepuzites | April 06 | April 14 | 68 % | 32 % | 25 % | 75 % | |||||
3Su ≻ Mar22 | April 05 | April 13 | 82¼% | 17¾% | 39¼% | 60¾% | |||||
3Su ≻ Mar21 | April 04 | April 12 | 96¾% | 3¼% | 53¾% | 46¼% | |||||
W14-7 | 094 | 102 | 100% | 57 % | 43 % | ||||||
Su ≻ Th ≻ Mar = 3Su ≻ Mar20 |
April 03 | April 11 | 10¾% | 89¼% | 67¾% | 32¼% | |||||
2Su ≻ Mar25 | April 01 | April 09 | 39¼% | 60¾% | 96¼% | 3¾% | |||||
14Su ≻ Dec | 091 | 099 | 43 % | 57 % | 100% | ||||||
Su ≻ Mar | March 31 | April 08 | 53¾% | 46¼% | 10¾% | 89¼% | |||||
W13-7 | 087 | 095 | 100% | 57 % | 43 % |
Mnemonic | after | before | ≤04 | 05 | 06 | 07 | 08 | 09 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | ≥18 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Western | 36.6% | 3.4% | 3.3% | 3.3% | 3.4% | 3.3% | 3.4% | 3.3% | 3.4% | 3.3% | 3.3% | 3.4% | 3.3% | 3.4% | 20.0% | ||
Eastern | 0.8% | 1.5% | 1.5% | 2.3% | 3.0% | 3.0% | 3.8% | 3.0% | 3.0% | 3.8% | 3.0% | 3.0% | 3.8% | 3.0% | 61.7% | ||
Common year | 10¾% | 11 % | 10¾% | 11 % | 10¾% | 10¾% | 10¾% | 10¾% | 11 % | 10¾% | 11 % | 10¾% | 10¾% | 10¾% | 10¾% | ||
Leap year | 3¼% | 3½% | 3½% | 3¼% | 3¾% | 3¼% | 3¾% | 3¼% | 3½% | 3½% | 3¼% | 3¾% | 3¼% | 3¾% | 3¼% | ||
Any year | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 14 % | ||
3Su ≻ Mar | April 14 | April 22 | 14½% | 14 % | 14½% | 57 % | |||||||||||
4Su ≻ Mar22 | April 12 | April 20 | 14¼% | 14¼% | 14½% | 14 % | 14½% | 28½% | |||||||||
4Su ≻ Mar21 | April 11 | April 19 | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 14 % | ||||||||
W15-7 | 101 | 109 | 3¼% | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 10¾% | |||||||
4Su ≻ Mar20 | April 10 | April 18 | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | ||||||||
Su ≻ 100 | 100 | 108 | 3¾% | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | 10¾% | |||||||
36Su ≺ Dec25 | April 09 | April 17 | 14½% | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | ||||||||
Su ≻ 099 | 099 | 107 | 3¼% | 14½% | 14 % | 14½% | 14¼% | 14¼% | 14½% | 10¾% | |||||||
Su ≻ 2Sa ≻ Mar | April 08 | April 16 | 14 % | 14½% | 14 % | 14½% | 14¼% | 14¼% | 14½% | ||||||||
15Su ≻ Dec | 098 | 106 | 3¾% | 14 % | 14½% | 14 % | 14½% | 14¼% | 14¼% | 10¾% | |||||||
World Calendar | 098 | 100 | 24¼% | 75¾% | |||||||||||||
2Su ≻ Mar | April 07 | April 15 | 14½% | 14 % | 14½% | 14 % | 14½% | 14¼% | 14¼% | ||||||||
Positivist Calendar | 097 | 099 | 24¼% | 75¾% | |||||||||||||
Pepuzites | April 06 | April 14 | 14¼% | 14½% | 14 % | 14½% | 14 % | 14½% | 14¼% | ||||||||
3Su ≻ Mar22 | April 05 | April 13 | 14¼% | 14¼% | 14½% | 14 % | 14½% | 14 % | 14½% | ||||||||
3Su ≻ Mar21 | April 04 | April 12 | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 14 % | ||||||||
W14-7 | 094 | 102 | 3¼% | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | 10¾% | |||||||
Su ≻ Th ≻ Mar = 3Su ≻ Mar20 |
April 03 | April 11 | 14 % | 14½% | 14¼% | 14¼% | 14½% | 14 % | 14½% | ||||||||
2Su ≻ Mar25 | April 1 | April 09 | 42½% | 14½% | 14¼% | 14¼% | 14½% | ||||||||||
14Su ≻ Dec | 091 | 099 | 46¼% | 14½% | 14¼% | 14¼% | 10¾% | ||||||||||
Su ≻ Mar | March 31 | April 08 | 57 % | 14½% | 14¼% | 14¼% | |||||||||||
W13-7 | 087 | 095 | 100% |
Some rules can be expressed in different but equivalent ways:
- The Sunday closest to the nth day.
- The first Sunday after the (n−4)th day.
- The last Sunday before the (n+4)th day.
- The Sunday in the range from the (n−3)th to the (n+3)th day.
Such an nth day is always relative to an arbitrary reference point:
- The start of the year (n ∈ DDD),
- the start of a particular calendar month (i.e. April)
- the end of a particular calendar month (i.e. March),
- an observed or tabulated astronomic event, i.e.
- (after) the equinox or (before) the solstice
- a particular lunar phase
- another holiday, e.g.
- the day after an eventual leap day.
The reference point can also be restricted to a certain day of the week, e.g. the Sunday after the nth Thursday of the year is the 7th day of the ordinal ISO week number n, “Wnn-7”. Since the leap day, 29 February, always comes before the spring equinox, there is an offset of 1 day possible for many rules.
- The Sunday (D = 7) of ISO week number WW is the one closest to DDD = 7×WW.
The following explanations are grouped by their normalized reference point.
The nth Sunday of the year (soy)[]
Realistic values for n are limited by the currently possible dates for Easter in the Gregorian calendar, hence:
- 12th Sunday, 078–084, 19–24 March (18 in leap, 25 in common years)
- 13th Sunday, 085–091, 26–31 March (25 in leap, 1 April in common years)
- 14th Sunday, 092–098, 2–7 April (1 in leap, 8 in common years)
- 15th Sunday, 099–105, 9–14 April (8 in leap, 15 in common years)
- 16th Sunday, 106–112, 16–21 April (15 in leap, 22 in common years)
- 17th Sunday, 113–119, 23–28 April (22 in leap, 29 in common years)
The middle ones, 14 and 15, are most popular among reform proponents. They are also the most frequent ones occurring. With the affordances mentioned below, this leaves just 15 and 16, because the earlier ones can be before the currently earliest Orthodox Easter date and the 17th can be later than the latest possible Catholic Easter date.
The Sunday after the nth weekday of the year (dow)[]
- Saturday: It arguably makes less sense to choose a Sunday after a certain Saturday than it makes for one before it.
- Monday: Such a Sunday is, in other words, the last day of the nth full week of the year.
- Thursday: This is equivalent to the ISO week of the year with the same ordinal number n.
With international standards, it would be simple to just define, say, W15-7 as Easter Sunday, but some Christian may disapprove of ISO 8601 simply because it uses Monday as the first day of the week and not Sunday. Similar to the Sunday of the year above, W15 and W16, but also (barely) W14 satisfy the requirements.
The Sunday after or before the nth day of the year (doy)[]
As it happens, the 100th day of the year is 9 April if it’s common and 8 April if it’s leap. This may be a definition of interest simply due to the nice round number.
For the World Calendar, day 099 (“8 April”) was deemed as a fixed date for Easter. Since 001 (“1 January”) is considered a Sunday and the start of the first week, this date would be the first day of the 15th week.
For the Positivist Calendar, day 098 (“14 Archimedes”) was deemed as a fixed date for Easter. Since 001 (“1 Moses”) is considered a Monday and the start of the first week, this date would be the last day of the 14th week, in the middle of the 4th month.
However, both of these calendar proposals break the ancient seven-day week cycle and are therefore unacceptable for most if not all Christian creeds.
To be agnostic of leap day offsets, it may be helpful to use the day of March as the Gaussian algorithm does: Gregorian Easter can fall on any of the 22nd through 56th DOMar.
The nth Sunday after the equinox or before the solstice[]
The Easter Computus assumes that the equinox is on 21 March in the calendar used. (This is almost always in W12, by the way.) It has been accepted practice for centuries now that this will not always be astronomically accurate for the Gregorian calendar and, due to its inadequate leap rule, has not been accurate anymore for the Julian calendar for much longer than a millennium. This nominal date is also called an ecclesiastical one. The following solstice is assumed to occur on 20 June (W25) which is also not always correct astronomically. Coincidentally, both events occur on the same day of the week, 13 weeks apart. Therefore, there are simple equivalent rules using either one of the as a fix point.
The same considerations for accracy are true for the date of the full moon, except that it’s still often correct in the Orthodox Computus because that is unaffected by its solar drift.
Since astronomic phenomena can be observed at a different time (be it civil or solar) in different places of the world, their actual date can also differ internationally. Furthermore, the start of a day and therefore date has had different definitions throughout history. The Abrahamitic religions traditionally begin a holiday in the evening of what secular people now would call the day before.
The acceptable dates would be the 14th through 35th day or the 3rd to 5th Sunday after the equinox.
The nth Sunday after or before Christmas[]
On 25 March, exactly 9 months before Christmas Day, there is a lesser Christian holiday called Feast of the Annunciation better known as Lady Day. Its celebration may be postponed if it occurs either anywhere within Good Week or Easter Week or at least for Maundy Thursday, Good Friday, Easter Sunday and Monday. To avoid this, a new fixed date for Easter could avoid this happening in the first place – in the most trivial case by never observing Easter before 2 April, which would much agree with Orthodox expectations. With a larger reform to the liturgical year, Lady Day could also be moved slightly, e.g. to exactly 40 weeks before Christmas Day.
The nth Sunday in April[]
- The 1st Sunday in April, i.e. 1–7 April, is slightly more often in W14 than W13. It is the 91st through 97th day of a common year or the 92nd through 98th day of a leap year.
- The 2nd Sunday is more often considered as a possible fixed date an better meets our expectations.
The Sunday after the nth weekday in April[]
- A popular proposal, especially in Great Britain and the Anglican Church apparently, is the Sunday after the 2nd Saturday in April. This would restrict the date range to 9–15 April, being almost always (96.25%) the 15th Sunday of the year (otherwise the 16th) and mostly (60.75%) in W15 (or else in W14). This is equivalent to the third Sunday after Lady Day.
- The Sunday of the 1st ISO week in April is the Sunday after the 1st Thursday in April, so on 4–10 April. Incidentally, this is also the 3rd Sunday after 20 March, which is a possible fixed date for the ecclesiastical Northward equinox. This is often but not always W14-7, sometimes it is W13-7.
13-month years[]
There are many calendar reform proposals that have 13 months of 4 weeks each. They differ in the choice of first day of the week and thereby month and by the treatment of the missing 365th day and the leap day or alternatively the 53rd or leap week or alternatively a 14th or leap month. Assuming that any such special calendar items are placed later than Easter, there are basically two designs:
- Sunday start
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- The 1st day of the 4th month is the 13th Sunday and the 85th day of the year.
- The 8th day of the 4th month is the 14th Sunday and the 92nd day of the year.
- The 15th day of the 4th month is the 15th Sunday and the 99th day of the year. This is within the ideal range.
- The 22nd day of the 4th month is the 16th Sunday and the 106th day of the year. This is within the ideal range.
- The 1st day of the 5th month is the 17th Sunday and the 113th day of the year.
- Sunday end
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- The 7th day of the 4th month is the 13th Sunday and the 91st day of the year.
- The 14th day of the 4th month is the 14th Sunday and the 98th day of the year. This is within the ideal range.
- The 21st day of the 4th month is the 15th Sunday and the 105th day of the year. This is within the ideal range.
- The 28th and last day of the 4th month is the 16th Sunday and the 112th day of the year. This is within the ideal range.
- The 7th day of the 5th month is the 17th Sunday and the 119th day of the year.
Equal-quarter years[]
Other calendar reform proposals have exactly 13 weeks, i.e. 91 days, per quarter; and some have exactly 90 days instead with more compensation elsewhere. They differ by the same features as 13-month years end also by the exact lengths of the months a quarter may be divided into. Easter is slightly more likely to be affected in these schemes, because it will almost always be placed early in the second quarter and special days or weeks may be placed at the edges of quarters.
Extension[]
If the Easter cycle had a more fixed nature, the whole Christian rites calendar could be optimized, especially the time between Christmastide and Lent, but the ember days, too, for instance.
To be more extreme, a proposal could try to reconcile with the lunisolar Jewish festivity calendar as well. For that, something like the International Lunation Calendar would be required and could then even cover Muslim and East-Asian traditional festivities determined in a common civic manner.