Calculating the date of Easter
n"The Council of
Nicaea (A.D. 325) set the date of Easter as the Sunday following the 14th day of the paschal full
moon, which is the full moon whose
14th day falls on or after the
vernal (spring) equinox. We know that Easter must always occur on a Sunday, because Sunday was the day of Christ's Resurrection. But why the 14th day of
the paschal full moon?
Because that was the date of Passover
in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, Easter was the Sunday after Passover."
- How Is the Date of Easter Calculated? By Scott P.
Richert
(http://catholicism.about.com/od/holydaysandholidays/f/Calculate_Date.htm)