Why does the date for Easter change every year? 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 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 in 325 AD, when it was established by the Council of Nicea.
However, the course of history has modified the meaning of this instruction, and therefore, a clearer, more accurate explanation is necessary today.
There are, in fact, as many misunderstanding about the calculation of Easter dates, as there are reasons for confusion about Easter dates. What follows is an attempt to clear up at least some of the confusion.
In actuality, the date of the Paschal Full Moon is determined from historical tables, and has no correspondence to lunar events. In the year 325 AD astronomers approximated the dates of all the full moons in the year for the Western Christian churches. These were called the Ecclesiastical Full Moon dates, and they have been used ever since 326 AD to determine the date of Easter. So, the Paschal Full Moon is always the first Ecclesiastical Full Moon date after March 20 (which happened to be the vernal equinox date in 325 AD).
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.
Western churches use the Gregorian Calendar to calculate the date of Easter and Eastern Orthodox churches use the Julian Calendar. This is partly why the dates are rarely the same.
Easter and its related holidays do not fall on a fixed date in either the Gregorian or Julian calendars, making them moveable holidays. The dates, instead, are based on a lunar calendar very similar to the Hebrew Calendar.
The Eastern Orthodox Church not only maintains the date of Easter based on the Julian Calendar which was in use during the First Ecumenical Council of Nicea in 325 AD, but also according to 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 325 AD. This means, in order to stay in line with the originally established (325 AD) vernal equinox, Orthodox Easter cannot be celebrated before April 3 (present day Gregorian calendar), which was March 21 in 325 AD.
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 death, burial and Resurrection of Christ happened after the celebration of Passover. Eventually the Orthodox Church came up with an alternative to calculating Easter based on Passover, and developed a 19-year cycle, as opposed to the Western Church 84-year cycle.
