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Q. How is the date of Easter determined every year?

Easter is considered a A. moveable feast and can fall anywhere from March 22 to April 25. Traditionally, Easter was celebrated on the first Sunday following the feast of Passover. Passover is celebrated on the 15th of the Hebrew month of Nisan, and should correspond to a full moon. However, the Christian church wished to eliminate any dependency on the Hebrew calendar, so they established a new way to determine the date of Easter each year.

In 325 CE, the Roman Emperor Constantine convened the First Council of Nicaea, who created a system for determining the date of Easter. According to Religious Holidays and Calendars: An Encyclopaedic Handbook by Aiden Kelly, Peter Dresser, and Linda M. Ross, “The Feast of the Resurrection should be celebrated on the first Sunday after (not on) the first full moon after March 21 (which was the date of the vernal equinox the year the council met). This full moon became known as Paschal.”

However, it is still more complicated than that. To say Easter occurs on the first Sunday after the full moon that happens next after the vernal equinox is not a precise statement of actual ecclesiastical rules. The Paschal full moon involved is not the astronomical full moon, but an ecclesiastical full moon. In 325 CE, astronomers approximated astronomical full moon dates for the Christian church, calling them ecclesiastical full moon dates (Kelly, et. al). These ecclesiastical full moons keep, more or less, in step with the astronomical full moon, but not always. Although modified slightly from its original form, by 1583 CE the table for determining ecclesiastical full moon dates was permanently established, and has been used ever since to determine the date of Easter.