The Western (Roman Catholic and Protestant) Christian churches use the Gregorian tables while many Eastern (Orthodox) Christian churches use older tables based on the Julian Calendar.Thus, the civil date of Easter depends upon which tables - Gregorian or pre-Gregorian - are used.
The Gregorian Calendar used by most Western Christian churches is the standard international calendar for civil use.
It also regulates the ceremonial cycle of the Roman Catholic and Protestant churches.
The ecclesiastical rules that determine the date of Easter trace back to 325 CE at the First Council of Nicaea convened by the Roman Emperor Constantine.
At that time the Roman world used the Julian Calendar (put in place by Julius Caesar).
For dates of Easter and other Christian observances, see the Selected Christian Observances service.
The lunar cycles used by the ecclesiastical system are simple to program.
By the 1700's, though, most of western Europe had adopted the Gregorian Calendar.
The Eastern Christian churches still determine the Easter dates using the older Julian Calendar method.
The Council decided to keep Easter on the same Sunday throughout the Christian world.