International Easter Calendar

The International Easter Calendar is a lunisolar week calendar. The first day of the week 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.

Weeks

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

Probabilities for the weeks of Catholic holidays dependent on Easter

Week Name	12.1%	23.5%	23.2%	23.3%	17.6%	0.3%
Fat	W05	W06	W07	W08	W09	W10
Carnival	W06	W07	W08	W09	W10	W11
…	…
Passa	W11	W12	W13	W14	W15	W16
Holy	W12	W13	W14	W15	W16	W17
Easter	W13	W14	W15	W16	W17	W18
…	…
Ascension	W18	W19	W20	W21	W22	W23
Whit	W19	W20	W21	W22	W23	W24
Pentecost	W20	W21	W22	W23	W24	W25
Corpus	W21	W22	W23	W24	W25	W26
Heart	W22	W23	W24	W25	W26	W27

Distribution of the calendar week of Easter

Distribution of the number of weeks between successive Easters

Since Easter is almost always in ISO week W13 through W17 (and very rarely in W18), the theoretical minimum year length is 48 weeks and the theoretical maximum is 57 weeks. In practice, however, successive Easters are always 50, 51, 54 or 55 weeks apart.

Distance between Easter dates, in weeks (1700–2299)

Weeks	Count	Ratio
48	0	0.0%
49	0	0.0%
50	141	23.5%
51	237	39.6%
52	0	0.0%
53	0	0.0%
54	37	6.2%
55	184	30.7%
56	0	0.0%
57	0	0.0%

Although Christian tradition has the week begin on "the Lord's day", i.e. Sunday, international rules as laid down in ISO 8601 require the first day of the week to be Monday and hence the week to end on Sunday.

A simplified rule for the distribution of year types could employ a 120-year cycle, e.g. 50, 51, 54 and 55-week years occurring with the following frequencies:

• 25% (30), 40% (48), 5⅚% (7) and 29⅙% (35) or
• 25⅚% (31), 39⅙% (47), 5% (6) and 30% (36).

Overview of week groupings 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					TBA					TBA					TBA					Hallow					Advent					Christmas					Epiphany					Passion
	Eastide							Trinitide							Ladytide							Angeltide							Adventide							Christide								Lentide
Short year with long month	Easter					Whit					TBA					TBA					TBA					Hallow					Advent					Christmas					Epiphany					Passion
	Eastide							Trinitide							Ladytide							Angeltide							Adventide								Christide								Lentide
Long year with short month	Easter					Whit					TBA					TBA					TBA					Hallow					Advent					Christmas					Epiphany					Leap				Passion
	Eastide							Trinitide								Ladytide								Angeltide								Adventide								Christide								Lentide
Long year without short month	Easter					Whit					TBA					TBA					TBA					Hallow					Advent					Christmas					Epiphany					Leap					Passion
	Eastide							Trinitide								Ladytide								Angeltide								Adventide								Christide								Lentide

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.

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	TBA	5	5	5	5
04	TBA	5	5	5	5
05	TBA	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.

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.

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