The Festal Letters or Paschal Letters are a series of annual letters by which the Archbishops of Alexandria, in conformity with a decision of the First Ecumenical Synod of Nicaea, announced the date on which Easter was to be celebrated. The Synod chose Alexandria because of its famous school of astronomy, and the date of Easter depends on the spring equinox and the phases of the moon.
The most famous of those letters are those authored by Athanasius, a collection of which was rediscovered in a Syriac translation in 1842. Because he served as Archbishop of Alexandria for forty-five years, he is said to have issued forty-five Paschal Letters, though because he was unable to do so for certain years due to him being in exile or away on episcopal matters, a number of these letters either were never written or are missing. Below are either the letters that have been preserved, either in their entirety or as fragments.
Read also: The Doctrine of Sanctification in St. Athanasius' Paschal Letters