International Easter Calendar

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

Week Name	12.1%	23.5%	23.2%	23.3%	17.6%	0.3%	Monday	Tuesday	Wednesday	Thursday	Friday	Saturday	Sunday
Fat	W05	W06	W07	W08	W09	W10				Fat			Shrove
Carnival	W06	W07	W08	W09	W10	W11	Rose	Pancake	Ash				Invocabit
…	…
Passa	W11	W12	W13	W14	W15	W16							Palm
Holy	W12	W13	W14	W15	W16	W17	Holy	Holy	Holy	Maundy	Good	Holy	Easter
Easter	W13	W14	W15	W16	W17	W18	Easter						Divine Mercy
…	…
Ascension	W18	W19	W20	W21	W22	W23				Ascension
Whit	W19	W20	W21	W22	W23	W24							Whitsun
Pentecost	W20	W21	W22	W23	W24	W25	Pentecost						Trinity
Corpus	W21	W22	W23	W24	W25	W26				Corpus Christi
Heart	W22	W23	W24	W25	W26	W27					Sacred Heart	Immaculate Heart

Distribution of the calendar week of Easter

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

• General Roman Calendar, Roman rite
• Liturgical year
• Easter Calendar
• Gregorian Lunar Calendar
• International Liturgical Calendar