Have you ever wondered why Easter Sunday can fall anywhere between March 22 and April 25? And why do Eastern Orthodox churches celebrate Easter on a different day than Western churches? These are all good questions with answers that require a bit of explanation. In fact, there are as many misunderstandings about the calculation of Easter dates, as there are reasons for the confusion. What follows is an attempt to clear up at least some of the confusion.
The Short Answer
At the heart of the matter lies a very simple explanation. The early church fathers wished to keep the observance of Easter in correlation to the Jewish Passover. Because the death, burial, and resurrection of Jesus Christ happened after the Passover, they wanted Easter to always be celebrated subsequent to the Passover. And, since the Jewish holiday calendar is based on solar and lunar cycles, each feast day is movable, with dates shifting from year to year. Now, from here the explanation grows more complicated.
The Long Answer
Today in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon date of the year. I had previously, and somewhat erroneously stated, "Easter is always celebrated on the Sunday immediately following the first full moon after the vernal (spring) equinox." This statement was true prior to 325 A.D.; however, over the course of history (beginning in 325 A.D. with the Council of Nicea), the Western Church decided to established a more standardized system for determining the date of Easter.
In actuality, the date of the Paschal Full Moon is determined from historical tables, and has no correspondence to lunar events.
As astronomers were able to approximate the dates of all the full moons in future years, the Western Christian Church used these calculations to establish a table of Ecclesiastical Full Moon dates. These dates would determine the Holy Days on the Ecclesiastical calendar.
Though modified slightly from its original form, by 1583 A.D. the table for determining the Ecclesiastical Full Moon dates was permanently established and has been used ever since to determine the date of Easter. Thus, according to the Ecclesiastical tables, the Paschal Full Moon is the first Ecclesiastical Full Moon date after March 20 (which happened to be the vernal equinox date in 325 A.D.). So, in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon.
The Paschal Full Moon can vary as much as two days from the date of the actual full moon, with dates ranging from March 21 to April 18. As a result, Easter dates can range from March 22 through April 25 in Western Christianity.