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.
At the heart of the matter lies a very simple explanation. The earliest believers in the church of Asia Minor wished to keep the observance of Easter in correlation with 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.
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 establish 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.
Historically, Western churches used the Gregorian Calendar to calculate the date of Easter, and Eastern Orthodox churches used the Julian Calendar. This was partly why the dates were seldom the same.
Easter and its related holidays do not fall on a fixed date in either the Gregorian or Julian calendars, making them movable holidays. The dates, instead, are based on a lunar calendar very similar to the Hebrew Calendar.
While some Eastern Orthodox Churches not only maintain the date of Easter based on the Julian Calendar, which was in use during the First Ecumenical Council of Nicea in 325 A.D., they also use the actual, astronomical full moon and the actual vernal equinox as observed along the meridian of Jerusalem. This complicates the matter, due to the inaccuracy of the Julian calendar, and the 13 days that have accrued since A.D. 325. This means, in order to stay in line with the originally established (325 A.D.) vernal equinox, Orthodox Easter cannot be celebrated before April 3 (present day Gregorian calendar), which was March 21 in A.D. 325.
Additionally, in keeping with the rule established by the First Ecumenical Council of Nicea, the Eastern Orthodox Church adhered to the tradition that Easter must always fall after the Jewish Passover, since the resurrection of Christ happened after the celebration of Passover. Eventually the Orthodox Church came up with an alternative to calculating Easter based on the Gregorian calendar and Passover, and developed a 19-year cycle, as opposed to the Western Church's 84-year cycle.
Since the days of early church history, determining the precise date of Easter has been a matter for continued argument. For one, the followers of Christ neglected to record the exact date of Jesus' resurrection. From then on, the matter grew increasingly complex.