Why does Easter occur at different times each year?
Easter, for Western churches, is calculated as the first Sunday after the paschal full moon that occurs on or after the vernal equinox. If the full moon falls on a Sunday, then Easter is the following Sunday. The holiday can occur anywhere between March 22 and April 25.
The Council of Nicaea in 325 C.E. established that Easter would be celebrated on Sundays; before that Easter was celebrated on different days in different places in the same year.
Read more about the dates of Western Easter, Orthodox Easter, and Passover in Moveable Feasts.
—The Fact Monster