Why the date of Easter changes every year
Easter ranks among the most prominent holidays in the western world, and is considered by many to be the most important holiday for Christians – even more important than Christmas.
Yet, Easter has no permanently fixed numeric date, like Christmas’ December 25th (in the Western Church), but seems to wander back and forth across the calendar from year to year with no apparent rhyme or reason.
While you don’t need to understand why Easter’s date varies in order to find the date on your calendar, for those with “curious minds,” we explain the reason for this date-shifting just below.
Reasons that the date of Easter changes
1. Passover is based on a lunar calendar
Those familiar with the story of the crucifixion as recorded in the Bible will know that Jesus died during the Jewish Passover Feast. He is said, in fact, to be the true “Passover Lamb of God” whose death paid for sins once for all, ending the need for further Passover sacrifices.
And as the original Passover, which was kept just before the Exodus of the Jewish people from slavery in Egypt, is considered a divinely appointed symbol, also called a “type” or “shadow,” of Christ, the ultimate and final Passover Lamb.
This Jewish historical background of Easter, which took place three days after Passover, led early Christians to try to keep the feast of Easter in coordination with the Jewish Passover. That date, however, was determined based on the Jewish calendar, which was a lunar calendar.
Since solar calendars differ from lunar ones by around 11 days per year, this made celebrating Easter on a solar calendar, such as Romans were using even in ancient times, very, very complicated.
This leads to our second point.
2. The Council of Nicea in A.D. 325
Originally, the whole Roman Empire used the Julian Calendar, which was instituted by Julius Caesar in 45 B.C. as a reform of an earlier Roman Calendar. As a solar calendar, it needed special rules to keep the lunar-based date of Easter determinable on it each year. This difficulty ultimately led such special rules being declared at the Council of Nicea in A.D. 325.
The council’s decision was to celebrate Easter on the first Sunday following the vernal equinox, which is the one day in spring when daytime and dark are of equal duration. This put the date around the time of Passover but adhered to the Julian instead of Jewish Calendar in determining it.
However, there are two further complications. First, the vernal equinox can occur anywhere from March 18th to March 21st. As it occurred on March 20th in A.D. 325, this became the fixed “Ecclesiastical Vernal Equinox,” which is technically what Easter’s date is based on rather than on the actual, astronomical vernal equinox.
The second complication is that a full moon occurs only for a moment and not for a full day. It also occurs at slightly different times in different time zones, and on a different day just across the International Date Line. Thus, a “Paschal Full Moon” schedule was established by the church, which does not always exactly correspond to the actual time of the full moon after the ecclesiastical spring equinox.
3. Gregorian vs. the Julian calendars
In 1582, since the Julian Calendar had gotten out of sync with the vernal equinox by 10 days, Pope Gregory XIII declared a new calendar to take its place. That calendar is named “Gregorian” after Pope Gregory and is used in the West to the present day.
The new calendar modified the leap-year rules to prevent further “day-drift” and decreed that the world would “leap” from October 5th to October 14th to make up for the “10 lost days.” This led to protests and resistance in some Protestant countries many years.
England didn’t get on board until 1753, and Germany waited even longer. And the Eastern Orthodox churches still hold to the Julian Calendar, which means that their date of Easter comes a week later usually, and sometimes it lands as late as May.