The Easter or Pascha date is determined by four main factors. The first two have been established by Apostolic canon, making them binding, and the last two have been established by church tradition.
The history of determining Easter dates has been complex and sometimes controversial, but the following pre-conditions provide a general summary.
The Easter date is set according to the Julian or Old Calendar and this date must be:
1) after the Vernal (Spring) Equinox, and 2) after the Jewish Passover, and 3) with the first full moon in Spring, and 4) on a Sunday after 21 March.