How is the date of Easter determined?
The rule most people remember is that Easter falls on the first Sunday after the first full Moon following the March equinox. In practice, Roman Catholic and Protestant churches follow a method of calculation adopted with the Gregorian calendar reform of 1582. The ecclesiastical dates of the March equinox (deemed to be March 21st) and full Moon do not always agree with those defined astronomically.
Normally the choice of definitions doesn’t matter, but there are rare years when Easter comes at an unexpected time. In 1981 Easter fell on April 19th, when it would have been April 26th, astronomically speaking. The next such discrepancy occurs in 2038, when, rather than falling on March 28th per the astronomical definition, Easter will take place four weeks later, on April 25th!
Easter is celebrated on March 27, 2005. Because of the lunar cycles involved, there are never successive years with Easter in March. In most years Easter falls in April. In fact, rarely is there an “isolated” April Easter — the last was in 1990, and the next will be in 2085, according to Jean Meeus’s Mathematical Astronomy Morsels (Willmann-Bell, 1997).
The next March Easter will occur in 2008, on the 23rd. Not since 1913 has there been an Easter Sunday as early as March 23rd, and it won’t happen again until 2160.
— Greg Bryant