Easter is based on the date of the first full moon in spring.
Easter is based on the date of the first full moon in spring.

Christians celebrate the resurrection of Jesus Christ on Easter. The date of Easter is different every year because it was originally based on the Jewish holiday of Passover, which falls on different days every year. Now, Easter is the first Sunday after the first full moon on or after the Vernal Equinox.

Finding the First Full Moon After the Equinox

March 21 is the official date of the Vernal Equinox, when the day and night are the same length, though the actual equinox may be a day or two off from that. So, the first step is to determine the first full moon on or after March 21. Since Easter is the first Sunday after that full moon, the earliest it could possibly be is March 22. That would only occur if there were a full moon on March 21 and it was a Saturday.

Calculating the First Sunday After the Full Moon

In 2008, Easter was on March 23 because the first full moon was on Equinox, or Friday, March 21. In 2018, the first full moon will be on March 31. Since that is a Saturday, Easter Sunday will be the following day, or April 1. Mathematically and astronomically, the latest Easter can be is April 25.