Computus (Latin for computation) is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age.
The canonical rule is that Easter day is the first Sunday after the 14th day of the lunar month (the nominal full moon) that falls on or after 21 March (nominally the day of the vernal equinox). For determining the feast, Christian churches settled on a method to define a reckoned “ecclesiastical” full moon, rather than observations of the true Moon. Eastern Orthodox Christians calculate the fixed date of 21 March according to the Julian Calendar rather than the modernGregorian Calendar, and use an ecclesiastical full moon that occurs four to five days later than the western ecclesiastical full moon.
In modern language, this definition is best described as: Easter is the Sunday following the Paschal Full Moon date. The Paschal Full Moon date is the Ecclesiastical Full Moon date following 20 March and, for the years 1900 to 2199, can be found in Tabular methods.
We here at Deskarati find it easier to pop this equation into Excel (Cell A1 needs to be the year) – Happy Easter