Page "Easter" Paragraph 53

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, 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.