Calculating the date of Easter
n"Though the New Testament contains no reference to an annual feast celebrating the Resurrection, the practice was well-established by the second century. Early churchmen were divided on whether to hold a feast on 14 Nisan (the date of the Biblical Pesach, which morphed into the name for Easter in many languages) or on the following Sunday; disputes and excommunications ensued in this Quartodeciman controversy until the Council of Nicea in 325 decided it must fall on a Sunday. Eventually the date was formulated roughly as "the first Sunday after the full moon that occurs on or after the vernal equinox." This can range between March 22 and April 25."

  - Easter, from Answers.com      (http://www.answers.com/topic/easter)