Have you ever wondered why Easter Sunday can fall anywhere between March 22 and April 25? And why Eastern Orthodox churches usually celebrate Easter on a different day than Western churches? These are good questions with answers that require a bit of explanation.

Why Does Easter Change Every Year?

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 that point on the matter only grew increasingly more complex.

A Simple Explanation

At the heart of the matter lies a simple explanation. Easter is a movable feast. The earliest believers in the church of Asia Minor wished to keep the observance of Easter correlated to the Jewish Passover. The death, burial, and resurrection of Jesus Christ happened after the Passover, so followers wanted Easter always to be celebrated after 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.

The Lunar Impact on Easter

Prior to 325 A.D., Easter was celebrated on the Sunday immediately following the first full moon after the vernal (spring) equinox. At the Council of Nicaea in 325 A.D., the Western Church decided to establish a more standardized system for determining the date of Easter.

Today in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon date of the year. The date of the Paschal Full Moon is determined from historical tables. The date of Easter no longer directly corresponds to lunar events. As astronomers were able to approximate the dates of all full moons in future years, the Western Church used these calculations to establish a table of Ecclesiastical Full Moon dates. These dates determine the Holy Days on the Ecclesiastical calendar.

Although 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.). Thus, 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.

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 Nicaea 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 the year A.D. 325. and 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 Nicaea, 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, by developing a 19-year cycle, as opposed to the Western Church's 84-year cycle.