Easter is an annual festival observed throughout the Christian world. The date for Easter shifts every
year within the Gregorian Calendar. The Gregorian Calendar is the standard international calendar for
civil use. In addition, it regulates the ceremonial cycle of the Roman Catholic and Protestant
churches. The current Gregorian 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).
The Council decided to keep Easter on a Sunday, the same Sunday throughout the world. To fix
incontrovertibly the date for Easter, and to make it determinable indefinitely in advance, the Council
constructed special tables to compute the date. These tables were revised in the following few
centuries resulting eventually in the tables constructed by the 6th century Abbot of Scythia,
Dionysis Exiguus. Nonetheless, different means of calculations continued in use throughout the Christian world.