This new calendar is referred to as the Gregorian Calendar.One major difference between the Julian and Gregorian Calendars is the "leap year rule".Easter is an annual festival observed throughout the Christian world.
In 1582, Christopher Clavius and a council working at the direction of Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian Calendar producing new Easter tables.
The new calendar was issued in February in the papal bull called "Inter gravissimas".
In some cases the difference may apply to some parts of the world and not to others because two different dates separated by midnight are always simultaneously in progress on the Earth.
For example, in 1962 the astronomical full moon occurred on March 21, UT=7h 55m - about six hours after astronomical equinox.
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 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.There are three major differences between the ecclesiastical and the astronomical systems.Inevitably, the date of Easter occasionally differs from a date that depends on the astronomical full moon and astronomical vernal equinox.For dates of Easter and other Christian observances, see the Selected Christian Observances service.