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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[1]
Week W06 W07 W08 W09 W10 W11 W12 W13 W14 W15 W16 W17 W18 W19 W20 W21 W22 W23 W24 W25
Carnival 12.1% 23.5% 23.2% 23.3% 17.6% 0.3%                            
Passa             12.1% 23.5% 23.2% 23.3% 17.6% 0.3%                
Easter               12.1% 23.5% 23.2% 23.3% 17.6% 0.3%              
Ascension                         12.1% 23.5% 23.2% 23.3% 17.6% 0.3%    
Pentecost                           12.1% 23.5% 23.2% 23.3% 17.6% 0.3%  
Corpus Christi                             12.1% 23.5% 23.2% 23.3% 17.6% 0.3%
Sum 12.1% 23.5% 23.2% 23.3% 17.6% 0.3% 12.1% 35.6% 46.7% 46.5% 40.9% 17.9% 12.4% 35.6% 58.8% 70.0% 64.1% 41.2% 17.9% 0.3%


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
Hebrew Nisan Iyar Sivan Tammuz Av Elul Tishrei Cheshvan Kislev Tevet Shevat Adar Leap

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[]

References[]

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