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Neela Kale Answers:
According to the formula of the Church established at the Council of Nicaea in the year 325, Easter is celebrated on the Sunday following the first full moon after the spring equinox. (Note that the ecclesiastical designation of the spring equinox and the full moon does not always correspond to the astronomical designation.) This date was chosen in keeping with the date of the Jewish Passover, from which Easter, the Christian Passover, was derived. Eastern and Western Church authorities have defined the spring equinox and the full moon differently, however, and since the Gregorian reform of the calendar, the date of Easter in the Eastern and Western Churches is not always the same. Following the formula, Easter can fall as late as April 25 or as early as March 22. In 2011, after the spring equinox on March 21, the full moon falls on April 18, so Easter falls on the following Sunday, April 24.
For a more detailed explanation of the calculations surrounding the date of Easter, see http://aa.usno.navy.mil/faq/docs/easter.php