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The International Easter Calendar (IEC) is a lunisolar week calendar. The first day of the year is Easter Monday, the day after Easter as determined by the Gregorian Computus. The number of the year is the same as in the Gregorian calendar for dates after Easter and one year less for dates 1 January through Easter.

The Easter Calendar is very similar, but has weeks starting on Sunday.

Weeks[]

Due to the lunisolar nature of the calendar, the length of the year varies between 50 and 55 weeks. There are no leap weeks per se, but the number of days per year is always divisible by seven.

Contrary to Christian tradition, where the week begins on "the Lord's day", i.e. Sunday, the international rules, as laid down in ISO 8601, require the first day of the week to be Monday and hence the week ends on Sunday. Therefore, Easter Week starts with Easter Monday and Holy Week ends with Easter Sunday.

Probabilities for the weeks of Catholic holidays dependent on Easter[1]
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


Easter calendar week

Distribution of the calendar week of Easter

Easter distance

Distribution of the number of weeks between successive Easters

Since Easter Sunday is almost always the last day of any ISO week between W12 and W16 (or very rarely W17), one might assume that the minimum year length was 48 weeks and the maximum was 57 weeks. In practice, however, successive Easters are always 50, 51, 54 or 55 weeks apart.

Distance between Easter dates, in weeks
Weeks 1700–2299 120-year alternative 1 120-year alternative 2
Count Ratio Count Ratio Count Ratio
50 141 23.5% 30 25% 31 25⅚%
51 237 39.6% 48 40% 47 39⅙%
54  37  6.2% 7 5⅚% 6 5%
55 184 30.7% 35 29⅙% 36 30%

Alternative, simplified rules for the distribution of year types could employ a 120-year leap cycle, with frequencies as shown in the table.

Subdivisions[]

Overview of week groupings (~5-week months and ~7-week tides) by type of year
Week 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 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
Short year without long month Easter Whit Solstice Tween Equinox Hallow Advent Christmas Epiphany Passion
Short year with long month Passion
Long year with short month Leap Passion
Long year without short month Leap Passion
Short year without long month Eastide Trinitide Ladytide Angeltide Adventide Christide Lentide
Short year with long month Adventide Christide Lentide
Long year with short month Trinitide Ladytide Angeltide Adventide Christide Lentide
Long year without short month Lentide
Hebrew* Nisan Iyar Sivan Tammuz Av Elul Tishrei Cheshvan Kislev Tevet Shevat Adar
Leap Adar
Kislev Tevet Shevat Adar
Leap Adar

Months[]

A short year consists of 10 months, a long year of 11 months. A normal month has 5 full weeks of 7 days each, a short months has just 4 weeks and a long month has 6 weeks. A single short month may occur in long years and a single long month may occur in short years. Both may be called special months. The additional month in long years is also called a leap month. The leap month is, when it occurs, the second-to-last month of the IEC year.

The first nine months of the year are always exactly 5 full weeks long.

The actual names of the months are up to the local language and tradition, although they are never the same as the names of the month in the Julian/Gregorian calendar. Some suggestions for nely coined terms in English: Eastember, Christember, Lentuary, Shrovuary.

Weeks per month by type of year
Month Short year without long month Short year with long month Long year with short month Long year without short month
01 Eastermonth 5 5 5 5
02 Whitmonth, Pentecostmonth 5 5 5 5
03 Solsticemonth, Lighmonth, Sunmonth 5 5 5 5
04 Betweenmonth, Tweenmonth 5 5 5 5
05 Equinoxmonth, Nightmonth 5 5 5 5
06 Hallowmonth 5 5 5 5
07 Adventmonth 5 5 5 5
08 Christmasmonth, Christmonth 5 5 5 5
09 Epiphanymonth 5 5 5 5
10 Leapmonth, Shrovemonth, Candlemasmonth, Candlemonth – (1) 4 5
10/11 Passionmonth, Lentmonth 5 6 (5) 5 5
Σ year 50 51 54 55

For some purposes, the long month of a short year may be counted as a single-week leap month.

The correspondence with Hebrew lunar month of 29 or 30 days is just approximate. In the table, they are shown lasting either 4 or 5 weeks exactly. It would be possible to have an alternative regular scheme with ten normal months of 4 weeks, whereof one may have a fifth leap week, and two longer months of 5 weeks and in addition a leap month of 4 weeks, e.g.: (4+5+4+4)+(4+4|5+4+4)+(4+5+4+4)+[4]

Tides[]

Each year, no matter the number of weeks or months in it, is divided into the same 7 tides. Each tide consists of either 7 or 8 full weeks. The only tides with a fixed length are the first of the year (Eastide) with always exactly 7 weeks, and the second-to-last tide (Christide) with always exactly 8 weeks. The second (Trinitide), third (Marytide) and fourth tide (Angeltide) are 7 weeks in short years and 8 weeks in long years. The fifth tide (Adventide) is usually 8 weeks long, but only 7 weeks in short years without a long month. The last tide (Lentide) is usually 7 weeks long, but 8 weeks in long years with a long month. The names of tides are subject to local languages, customs and traditions.

Weeks per tide by type of year
Tide Short year without long month Short year with long month Long year with short month Long year without short month
1 Eastertide, Eastide 7 7 7 7
2 Trinitytide, Trinitide 7 7 8 8
3 Marytide, Ladytide 7 7 8 8
4 Angeltide, Michaelmastide 7 7 8 8
5 Advent, Adventide 7 8 8 8
6 Christmastide, Christide 8 8 8 8
7 Lent, Lentide 7 7 7 8
Σ year 50 51 54 55

In this calendar, Christmas is a movable feast and if a late Easter follows, it may actually fall before Christmastide. Christmastide is subdivided into 2 weeks of Twelvetide and 6 weeks of Epiphanytide.

Weeks[]

Some weeks have special names.

  • The first week of the year is called Easter Week.
  • The first week of Trinitide is called Whitweek.
  • The first week of Lent is called Shrove Week or Carnival.
  • The final week of the year is called the Holy Week or Maundy Week.

See also[]

References[]

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