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.
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.
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.
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).
|Short year without long month||Easter||Whit||TBA||TBA||TBA||Hallow||Advent||Christmas||Epiphany||Passion||—|
|Short year with long month||Easter||Whit||TBA||TBA||TBA||Hallow||Advent||Christmas||Epiphany||Passion||—|
|Long year with short month||Easter||Whit||TBA||TBA||TBA||Hallow||Advent||Christmas||Epiphany||Leap||Passion||—|
|Long year without short month||Easter||Whit||TBA||TBA||TBA||Hallow||Advent||Christmas||Epiphany||Leap||Passion|
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.
|Month||Short year without long month||Short year with long month||Long year with short month||Long year without short month|
|10||Leapmonth, Shrovemonth, Candlemasmonth, Candlemonth||–||– (1)||4||5|
|10/11||Passionmonth, Lentmonth||5||6 (5)||5||5|
For some purposes, the long month of a short year may be counted as a single-week leap month.
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.
|Tide||Short year without long month||Short year with long month||Long year with short month||Long year without short month|
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.
- General Roman Calendar, Roman rite
- Liturgical year
- Easter Calendar