Calculating the date of Easter
n“Easter Sunday typically falls on a different date each year. Easter is always the first Sunday after or on the first full moon, after the spring (vernal) Equinox. The first Ecumenical Council was held at Nicea in present-day Turkey in the year 325. It decreed that Easter would be celebrated on the Sunday following the first full moon that occurred after the spring equinox. This retained a lunar connection as a sort of 'memory' of the Jewish calendar system, and ensured that the feast would be on a Sunday. Because lunar phases occur independently of the solar year, this means that there is a 'window' of several weeks during which Easter may be celebrated. By this reckoning, in our calendar, Easter must occur between March 22 and April 25.”

-  How is the Date for Easter Calculated, From WikiAnswers        (http://wiki.answers.com/Q/How_is_the_date_for_Easter_calculated_each_year)