Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date for the year. (Paschal is pronounced "PAS-KUL", not "pas-chal").
In June 325 A.D. astronomers approximated astronomical full moon dates for the Christian church, calling them Ecclesiastical Full Moon (EFM) dates.
- From 326 A.D. the PFM date has always been the EFM date after March 20 (which was the equinox date in 325 A.D.)
Easter Sunday is the date of the annual celebration of Christ's resurrection. The aim of the Easter Dating Method is to maintain, for each Easter Sunday, the same season of the year and the same relationship to the preceding astronomical full moon that occurred at the time of his resurrection in 30 A.D.
This was achieved in 1583 .D. using skill and common-sense by Pope Gregory the 13th, and his astronomers and mathematicians, predominantly Lilius and Clavius, by introducing their new larger (revised) PFM Gregorian dates table. This replaced the (original) 326 A.D. "19 PFM dates" table in the Julian calendar.
Easter Sunday, from 326 A.D., is always one of the 35 dates March 22 to April 25.
From 31 A.D. to 325 A.D. Easter Day was celebrated either:
(a) on or just after the first day of the Jewish Passover (no matter on which day of the week that Easter Day occurred), or
(b) on a Sunday close to or on the first Passover Day.
Both of these methods existed continuously throughout this period.
From 326 A.D. to 1582 A.D. Easter Sunday date was based on the Julian calendar in use at that time. It became defined as the Sunday following the Paschal Full Moon date for the year, using a simple "19 PFM dates" table.
The Julian calendar was replaced by the Gregorian calendar in October 1582 to re-align March 20 (and therefore Easter) with the seasons by removing 10 dates October 5 to 14, 1582. This replacement did not occur until later in many countries e.g. in September 1752 in England.
The Gregorian calendar very closely maintains the alignment of seasons and calendar dates by having leap years in only 1 of every 4 century years, namely, those divisible exactly by 400. One additional February 29 date will need to be removed in about 4140 A.D., therefore Easter calculations will need to use the changed Days of Week of PFM dates when the exact year for this removal is decided.
From 326 A.D., the Easter Sunday Date for any given year is NOT determined by the March Equinox date for that year. March 20 (not March 21) is the most common Gregorian Equinox date from 1583 to 4099 A.D.
Historically, references to March 21 have caused mistakes in calculating Easter Sunday dates. March 20 has become the important date in recent Easter dating methods. Despite frequent references to March 21, this date has no special significance to any recent Easter dating methods.