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.
Easter is a transitioning holiday and the exact date for it is calculated using the lunar-solar calendar. The day of Easter is taken to be on the first Sunday following the first full moon after the vernal equinox. And because the vernal equinox usually occurs on the 20th of March and two full moons are a little under 30 days apart, Easter usually happens either at the beginning or at the end of April. Although there have been cases when the Easter holiday occurred at the begging of May, or at the end of March.
Eastern vs. Western Easter Dates
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 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.