The current rules that determine the date of Easter originated in A.D. 325 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, based on a set of ecclesiastical rules, were revised in the following few centuries, resulting eventually in the tables constructed by the 6th century Abbot of Scythia, Dionysius Exiguus. Nonetheless, different means of calculation still persisted 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 that time, however, the cumulative mismatch of the Julian calendar year against the solar year totaled 10 days. Gregory's papal decree easily solved this problem by advancing the Julian calendar by 10 days. Thus, the calendar days October 5 - 14, 1582 simply vanished under the new Gregorian calendar.

Universal adoption of the Gregorian calendar occurred slowly. By the 1700's, though, most of Western Europe had embraced it and determined the date of Easter accordingly.

But the usual understanding—Easter Day is the first Sunday after the full moon that occurs next after the vernal equinox—is not a precise statement of the actual ecclesiastical rules. The full moon involved is not the astronomical full moon but an ecclesiastical moon (determined from tables) that keeps, more or less, in step with the astronomical Moon. But differences do arise from time to time.

According to the rules, Easter falls on the first Sunday following the first ecclesiastical full moon that occurs on or after the day of the vernal equinox. This particular ecclesiastical full moon is the 14th day of a tabular lunation (new moon) with the vernal equinox fixed as March 21.

Consequently, Easter can never occur before March 22 or later than April 25. But the civil date of Easter depends upon which tables—Gregorian or pre-Gregorian—are used. The Western (Roman Catholic and Protestant) Christian churches use the Gregorian tables; many Eastern (Orthodox) Christian churches use the older tables based on the Julian calendar, which is now 13 days behind the Gregorian calendar.

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 churches.

But Easter creates a more serious problem: what to do with a Man who was crucified—but then presented Himself alive by many convincing proofs over a period of 40 days (Acts 1:3) and was seen by more than 500 people at one time (1 Corinthians 15:6). Is He, as He claimed (John 14:6), the only way to God? And what about His assertion (John 3:3) that only those who have been born again will see the kingdom of God?

And unlike the decree that established the date of Easter, a personal decision about the Man of Easter must be exercised by each individual. What will we do with Jesus?

(Note: most of the material on the date of Easter was taken from the U.S. Naval Observatory web site.)