Easter Day can occur anywhere between March 22 and April 25. Often Easter Day for the Eastern half of the church is on a different date than for the Western half of the church. This year Eastern and Western Easter falls on the same date: Sunday April 4. Passover this year (5769) commences at sunset of Monday, the 29th of March.
The dating of Easter arises from the complicated joining of two different calendar systems. These calendars might be illustrated by the early story of Cain and Abel. If you are an Abel type – hunting, fishing, watching your flock by night, you will focus on the moon and the lunar cycle of 29 and a half days. Moonlight and tides will be significant to you. If you are a Cain type, a tiller of the ground and grower of crops, the solar cycle and its seasons will be more significant to you. The dating of Easter comes out of combining these solar and lunar calendars.
Passover – Pesach
The Jewish calendar is lunar. Twelve lunar months, with an occasional extra month popped in to keep up with the solar year. The month begins with new moon, and full moon, in the middle of the month, is the obvious time for extensive parties and festivals. There’s more light at night! Passover (Pesach) is the first full moon after the vernal (Northern Spring) equinox (14 of Abib in the Old Testament’s Hebrew Calendar) (Lev 23:5). This was to be a “perpetual ordinance” (Exodus 12:14).
Nicaea on Easter
There appears confusion between the synoptics (Matthew, Mark, and Luke) and John on the relationship between the Passover celebration and Christ’s death. Added to that, some early Christians celebrated Pascha (Easter) on the Jewish festival of Passover, whilst others always celebrated it on the Sunday following. The former were called Quartodecimans (Latin: quarta decima, fourteen). The Council of Nicaea (325) decided against the Quartodecimans and in favour of Easter always being on a Sunday. Rather than produce a canon on this, they communicated this to the different dioceses and gave the Bishop of Alexandria the privilege of announcing annually the date of Easter.
The Council of Nicaea determined that Easter would be the Sunday which follows the first full moon after the vernal equinox. If full moon happens to fall on Sunday, Easter is celebrated the following Sunday. Furthermore, it fixed the vernal equinox to be 21 March.
By the sixth century complex mathematical methods had been devised, involving paschal cycles of 19 years in the East, and 84 years in the West. Hence Easter calculations are based not on the astronomical full moon but an “ecclesiastical moon,” based on these created tables.
The Gregorian Calendar
A further divergence developed when there was a growing realisation of the drifting of the Julian Calendar from the actual solar year. In the Julian Calendar every year divisible by four is a leap year. This actually makes the Julian year slightly too long. By the sixteenth century this drift had made 10 days of difference. Pope Gregory in 1582 declared that October 4th would be followed by October 15th, and that only centuries divisible by 400 would be a leap year. This is known as the Gregorian calendar most now use. 1900 was not a leap year, but 2000 was a leap year. England, not under the authority of the pope, did not change to the Gregorian calendar until 1752 and there were riots demanding the giving back of the (by then) 12 days lost! The Eastern part of the church continues to calculate its festivals by the Julian calendar.
Hence, Eastern Easter also falls on a Sunday between March 22 and April 25, inclusive, but of the Julian calendar. There is currently a 13 day difference between the Julian and Gregorian calendars hence, from the Gregorian perspective, Eastern Easter falls between April 4 and May 8, inclusive. Eastern Easter also never comes before the Jewish Passover.
Easter in the future?
There has been discussion about abandoning any relationship with the lunar cycle and fixing Easter on the Sunday after the second Saturday in April. The Second Vatican Council agreed to a fixed date for Easter provided a consensus could be reached among Christian churches.
There was an ecumenical meeting in Aleppo, Syria in 1997. This concluded that the present differences in the calendars and lunar tables (paschal cycles) have no different fundamental theological outlook. The suggestion there was to replace both Eastern and Western calculations with the most advanced astronomically accurate calculations of the equinox and the full moon following, using the meridian of Jerusalem as the point of measure. This has not advanced further.