"The Sunday following the full Moon which falls on or after the equinox will give the lawful Easter."
Easter is determined on the basis of lunisolar cycles. The lunar year consists of 30-day and 29-day lunar months, generally alternating, with an embolismic month added periodically to bring the lunar cycle into line with the solar cycle. In each solar year (January 1 to December 31 inclusive), the lunar month beginning with an ecclesiastical new moon falling in the 29-day period from March 8 to April 5 inclusive is designated as the paschal lunar month for that year. Easter is the 3rd Sunday in the paschal lunar month, or, in other words, the Sunday after the paschal lunar month's 14th day.
The Gregorian calculation of Easter was based on a method devised by the Calabrian doctor Aloysius Lilius (or Lilio) for adjusting the epacts of the moon,[39] and has been adopted by almost all Western Christians and by Western countries who celebrate national holidays at Easter. For the British Empire and colonies, a determination of the date of Easter Sunday using Golden Numbers and Sunday letters was defined by the Calendar (New Style) Act 1750 with its Annexe. This was designed to exactly match the Gregorian calculation.
