Why does the date for Easter change every year?

Here’s a pop quiz: What date does Christmas fall on every year? Valentine’s Day? St. Patrick’s Day? Halloween? What about Easter?

If you were on a roll until that last question, you’re not alone, and it’s not your fault. You probably already know that Easter falls on a different date every year. But why? Here are some reasons as to why this happens.

Some people believe, Easter and many other church holidays related to it — such as Ash Wednesday, Palm Sunday, Good Friday — are called “moveable feasts,” because they do not fall on a fixed date on the Gregorian calendar, which follows the cycle of the sun and the seasons. Instead, these days follow a lunisolar calendar, similar to Jewish holidays.

According to a Fourth Century ruling, the date of Easter is set for on the first Sunday following the Paschal Full Moon, which is the first Full Moon of Spring (Autumn in the Southern Hemisphere), occurring on or shortly after the Vernal Equinox. March 22 is the earliest Easter can occur on any given year, and April 25 is the latest.

Some churches, primarily Eastern Orthodox Churches, use the older Julian calendar for holidays. Easter sometimes falls on different dates, according to that calendar.

Others believe, the earliest believers in the church of Asia Minor wished to keep the observance of Easter in correlation to 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.

While some  believe, in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon date of the year. Easter is always believed to be 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 established 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 could 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.

(With thanks to Brother Daniel for the historical input)