exvigilare

Last time, we talked about the wide range of dates on which Easter/Pascha can fall.

Now we ask the question, “why?” Why is there such a variation of dates for Easter/Pascha? At the First Council of Nicaea in 325 it was decided that all Christians would celebrate Easter on the same day, which would be a Sunday. It is probable that no method of determining the date was specified by the Council. The Council gave the Bishop of Alexandria the privilege of announcing annually the date of Christian Passover.

The formula for Easter—”The first Sunday after the first full moon on or after the vernal equinox”—is identical for both Western and Orthodox Easters, but the churches base the dates on different calendars: Western churches use the Gregorian calendar, the standard calendar for much of the world, and Orthodox churches use the older, Julian calendar.

In the Western Church, Easter sometimes precedes Passover by weeks. For the Orthodox, that can’t happen, because the Crucifixion and Resurrection of Christ took place after he entered Jerusalem to celebrate Passover.

Part of the issue is that the Western church does not use the actual, or astronomically correct date for the vernal equinox, but a fixed date , March 21. In 1997 the Council of World Churches proposed a solution thought to be favorable to both East and West. In 1963 the Second Vatican Council agreed to attempt to move the holiday to a fixed date, provided a consensus could be reached among Christian churches (that didn’t happen). The second Sunday in April has been suggested as the most likely date.

Back to the “why,”

The rule has since the Middle Ages been phrased as Easter is observed on the Sunday after the first full moon on or after the day of the vernal equinox. However, this does not reflect the actual ecclesiastical rules precisely. One reason for this is that the full moon involved (called the Paschal full moon) is not an astronomical full moon, but an ecclesiastical moon. Another difference is that the astronomical vernal equinox is a natural astronomical phenomenon, while the ecclesiastical vernal equinox is a fixed March 21. Easter is determined from tables which determine Easter based on the ecclesiastical rules described above, which approximate the astronomical full moon.

In applying the ecclesiastical rules, the various Christian Churches use 21 March as their starting point from which they find the next full moon, etc. However because Eastern Orthodox and Oriental Orthodox Churches use the Julian Calendar as their starting point, while Western Christianity uses the Gregorian Calendar, the end point, the date for Easter, may diverge.

A good chunk of this came from this and this article. As with most things about religious specifics, I find writings for children the best sources.