Why does the Easter holiday fall on a different day every year?
Two concepts play an important role when calculating the Paschal full moon: The Golden Number and the Epact. They are described in the following sections. The following sections give details about how to calculate the date for Easter. Note, however, that while the Julian calendar was in use, it was customary to use tables rather than calculations to determine Easter. The following sections do mention how to calculate Easter under the Julian calendar, but the reader should be aware that this is an attempt to express in formulas what was originally expressed in tables. The formulas can be taken as a good indication of when Easter was celebrated in the Western Church from approximately the 6th century.
In 325, at the Council of Nicæa the date of Easter was fixed by declaring that Easter should be the first Sunday after the first full moon occuring on or after the vernal equinox. This declaration was intended to be the end of a long dispute between eastern and western Christian communities. Fixing the date of Easter exactly was almost impossible at that time, the vernal equinox and the full moons having had to be determined in advance. Therefore, Easter was computed using a cyclic calculation which made it necessary to fix the vernal equinox and to create a lunar calendar for predicting full moons. The Council of Nicæa gave the task of calculating the date of Easter to the Church of Alexandria, which was to communicate the result to the pope for further distribution to the Christian communities. The computation described in this article is the one used by the Roman church since its introduction in the 6th century by Dionysius Exiguus. There are some slight differences between these ru
