Yes, I know it’s a little late for this question, as the holiday was just this past Sunday, but inquiring minds want to know, and it’s my job to find out. We know that Easter is always on a Sunday in the spring, but how do we know which Sunday that will be? We could always answer that by taking a quick glance at the calendar, but why is it that one year, Easter is on a Sunday in March and the next, it is on a Sunday at the tail end of April?
The date of Easter may appear to be random, but it’s actually determined by the lunar calendar. Easter is celebrated the Sunday after the full moon that occurs on or following the vernal equinox (the first day of spring). If that full moon happens to fall on a Sunday, Easter is held the next Sunday. This is the way we’ve determined the date of Easter since 325 A.D., when the First Council of Nicea met. The council was a meeting of Christian bishops who convened in present-day Turkey to resolve disagreements within the religion. One of the results of the bishops’ efforts? The standard formula for calculating the date of Easter, which is the one we still use today.
Source: Credo Reference