Easter is an
annual festival observed throughout the Christian
world. The date for Easter shifts every year within
the Gregorian Calendar. The Gregorian Calendar is
the standard international calendar for civil use.
In addition, it regulates the ceremonial cycle of
the Roman Catholic and Protestant churches. The
current Gregorian ecclesiastical rules that
determine the date of Easter trace back to 325 CE
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
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 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
In 1582 Gregory XIII (Pope of the Roman Catholic
Church) completed a reconstruction of the Julian
calendar and produced new Easter tables. One major
difference between the Julian and Gregorian
Calendar is the "leap year rule". See our FAQ on
Calendars for a description of the difference.
Universal adoption of this Gregorian calendar
occurred slowly. 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
The usual statement, that 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
The ecclesiastical rules are:
- Easter falls
on the first Sunday following the first
ecclesiastical full moon that occurs on or after
the day of the vernal equinox;
particular ecclesiastical full moon is the 14th
day of a tabular lunation (new moon); and
- the vernal
equinox is fixed as March 21.
that Easter can never occur before March 22 or
later than April 25. The Gregorian dates for the
ecclesiastical full moon come from the Gregorian
tables. Therefore, the civil date of Easter depends
upon which tables - Gregorian or pre-Gregorian -
are used. The western (Roman Catholic and
Protestent) Christian churches use the Gregorian
tables; many eastern (Orthodox) Christian churches
use the older tables based on the Julian
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.
There are three major differences between the
ecclesiastical system and the astronomical system.
- The times of
the ecclesiastical full moons are not
necessarily identical to the times of
astronomical Full Moons. The ecclesiastical
tables did not account for the full complexity
of the lunar motion.
- The vernal
equinox has a precise astronomical definition
determined by the actual motion of the Sun. It
is the precise time at which the apparent
longitude of the Sun is zero degrees. This
precise time shifts within the civil calendar
very slightly from year to year. In the
ecclesiastical system the vernal equinox does
not shift; it is fixed at March 21 regardless of
the actual motion of the Sun.
- The date of
Easter is a specific calendar date. Easter
starts when that date starts for your local time
zone. The vernal equinox occurs at a specific
date and time all over the Earth at once.
then, the date of Easter occasionally differs from
a date that depends on the astronomical Full Moon
and vernal equinox. In some cases this difference
may occur in some parts of the world and not in
others because two dates separated by the
International Date Line are always simultaneously
in progress on the Earth.
For example, take the year 1962. In 1962, the
astronomical Full Moon occurred on March 21, UT=7h
55m - about six hours after astronomical equinox.
The ecclesiastical full moon (taken from the
tables), however, occured on March 20, before the
fixed ecclesiastical equinox at March 21. In the
astronomical case, the Full Moon followed its
equinox; in the ecclesiastical case, it preceeded
its equinox. Following the rules, Easter,
therefore, was not until the Sunday that followed
the next ecclesiastical full moon (Wednesday, April
18) making Easter Sunday, April 22.
Similarly, in 1954 the first ecclesiastical full
moon after March 21 fell on Saturday, April 17.
Thus, Easter was Sunday, April 18. The astronomical
equinox also occurred on March 21. The next
astronomical Full Moon occurred on April 18 at
UT=5h. So in some places in the world Easter was on
the same Sunday as the astronomical Full Moon.
Following are dates of Easter from 2013 to 2023: