Easter is an annual festival observed throughout the Christian world. The date for Easter shifts every year within the Gregorian Calendar. The Gregorian Calendar is the standard international calendar for civil use. In addition, it regulates the ceremonial cycle of the Roman Catholic and Protestant churches. The current Gregorian ecclesiastical rules that determine the date of Easter trace back to 325 CE at the First Council of Nicaea convened by the Roman Emperor Constantine. At that time the Roman world used the Julian Calendar (put in place by Julius Caesar).
The Council decided to keep Easter on a Sunday, the same Sunday throughout the world. To fix incontrovertibly the date for Easter, and to make it determinable indefinitely in advance, the Council constructed special tables to compute the date. These tables were revised in the following few centuries resulting eventually in the tables constructed by the 6th century Abbot of Scythia, Dionysis Exiguus. Nonetheless, different means of calculations continued in use throughout the Christian world.
In 1582 Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian calendar and produced new Easter tables. One major difference between the Julian and Gregorian Calendar is the "leap year rule". Universal adoption of this Gregorian calendar occurred slowly. By the 1700's, though, most of western Europe had adopted the Gregorian Calendar. The Eastern Christian churches still determine the Easter dates using the older Julian Calendar method.
The usual statement, that Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox, is not a precise statement of the actual ecclesiastical rules. The full moon involved is not the astronomical Full Moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical Moon.
The ecclesiastical rules are:
resulting in that Easter can never occur before March 22 or later than April 25. The Gregorian dates for the ecclesiastical full moon come from the Gregorian tables. Therefore, the civil date of Easter depends upon which tables - Gregorian or pre-Gregorian - are used. The western (Roman Catholic and Protestant) Christian churches use the Gregorian tables; many eastern (Orthodox) Christian churches use the older tables based on the Julian Calendar.
In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern churches.
There are three major differences between the ecclesiastical system and the astronomical system.
Inevitably, then, the date of Easter occasionally differs from a date that depends on the astronomical Full Moon and vernal equinox. In some cases this difference may occur in some parts of the world and not in others because two dates separated by the International Date Line are always simultaneously in progress on the Earth.
For example, take the year 1962. In 1962, the astronomical Full Moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox. The ecclesiastical full moon (taken from the tables), however, occurred on March 20, before the fixed ecclesiastical equinox at March 21. In the astronomical case, the Full Moon followed its equinox; in the ecclesiastical case, it preceded its equinox. Following the rules, Easter, therefore, was not until the Sunday that followed the next ecclesiastical full moon (Wednesday, April 18) making Easter Sunday, April 22.
Similarly, in 1954 the first ecclesiastical full moon after March 21 fell on Saturday, April 17. Thus, Easter was Sunday, April 18. The astronomical equinox also occurred on March 21. The next astronomical Full Moon occurred on April 18 at UT=5h. So in some places in the world Easter was on the same Sunday as the astronomical Full Moon.
The following are dates of Easter from 1980 to 2024:
1980 April 6 1995 April 16 2010 April 4 1981 April 19 1996 April 7 2011 April 24 1982 April 11 1997 March 30 2012 April 8 1983 April 3 1998 April 12 2013 March 31 1984 April 22 1999 April 4 2014 April 20 1985 April 7 2000 April 23 2015 April 5 1986 March 30 2001 April 15 2016 March 27 1987 April 19 2002 March 31 2017 April 16 1988 April 3 2003 April 20 2018 April 1 1989 March 26 2004 April 11 2019 April 21 1990 April 15 2005 March 27 2020 April 12 1991 March 31 2006 April 16 2021 April 4 1992 April 19 2007 April 8 2022 April 17 1993 April 11 2008 March 23 2023 April 9 1994 April 3 2009 April 12 2024 March 31The rule is that Easter is the first Sunday after the first ecclesiastical full moon that occurs on or after March 21. The lunar cycles used by the ecclesiastical system are simple to program. The following algorithm will compute the date of Easter in the Gregorian Calendar system.
The algorithm uses the year, y, to give the month, m, and day, d, of Easter. The symbol * means multiply.
Please note the following: This is an integer calculation. All variables are integers and all remainders from division are dropped. For example, 7 divided by 3 is equal to 2 in integer arithmetic.c = y / 100 n = y - 19 * ( y / 19 ) k = ( c - 17 ) / 25 i = c - c / 4 - ( c - k ) / 3 + 19 * n + 15 i = i - 30 * ( i / 30 ) i = i - ( i / 28 ) * ( 1 - ( i / 28 ) * ( 29 / ( i + 1 ) ) * ( ( 21 - n ) / 11 ) ) j = y + y / 4 + i + 2 - c + c / 4 j = j - 7 * ( j / 7 ) l = i - j m = 3 + ( l + 40 ) / 44 d = l + 28 - 31 * ( m / 4 )
For example, using the year 2010,
n=2010 - 19 x (2010/19) = 2010 - 19 x (105) = 15, [see note above regarding integer calculations]
etc. resulting in Easter on April 4, 2010.