(from the Episcopal Book of Common Prayer):
Easter Day is always the Sunday after the full moon that occurs on or after the spring equinox on March 21. This full moon may happen on any date between March 21 and April 18 inclusive. If the full moon falls on Sunday, Easter Day is the Sunday following. But Easter Day cannot be earlier than March 22 or later than April 25.
I have never understood why it didn’t just happen on the Sunday after Passover.
Addendum: Easter Day this year is the earliest it has been since 1913 and will not fall as early for another 220 years.