According to the English book of Common Prayer, "Easter Day is the first Sunday after the full moon which happens upon, or next after the 21st day of March; and if the full moon happens upon a Sunday, Easter Day is the Sunday after."
Why such an odd definition? March 21 is the vernal equinox -- the day on which the length of daylight equals the length of darkness as the days are lengthening in the spring. The traditional Jewish calendar is based on moon phases, which is how the phase of the moon enters into the definition -- when they were determining what day Easter would fall on, they deferred to the Jewish practice of using moon phases to decide the timing of holidays.
Using this method, Easter can only occur between March 22 and April 25.
Here are some interesting links: