Setting the date for Easter

Christmas is December 25th, every year, but Easter moves dates from year to year. Ever wonder why that is? Hopefully, this will help to answer that question:
The date for Easter shifts every year within the Gregorian Calendar. The current Gregorian ecclesiastical rules that determine the date of Easter trace back to 325 AD 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 of Nicaea 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.

In 1582 Gregory XIII (Pope of the Roman Catholic Church) completed a reconstruction of the Julian calendar and produced new Easter tables. 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 usual statement is that Easter Day (Sunday) is the first Sunday after the first full moon that occurs next after the vernal equinox. Very few times has there been an exception to this rule. The result is that (generally) Easter can never occur before March 22 or later than April 25.

In a congress held in 1923, the eastern churches adopted a modified Gregorian Calendar and decided to set the date of Easter according to the astronomical Full Moon for the meridian of Jerusalem. However, a variety of practices remain among the eastern orthodox churches.