The date of Easter is normally defined as "The first Sunday after the first full moon after the Vernal Equinox." Sounds simple, but there are some real problems with it. One of the problems is: which longitude on earth are we talking about? At the precise instant of the full moon, it can be Sunday in New Zealand and Saturday in New York, which, if each followed their own longitude, would result in a different date of Easter in those two places. In addition, there is a problem with simply knowing what the moon's orbit is. The moon's orbit is subject to variation, and it was not possible to calculate it precisely until recently. Without the ability to precisely determine the moon's orbit, it was possible that the date of Easter would change as our technical ability to determine the moon's orbit changed.

To avoid these problems, at the Council of Nicea (AD 325) the Church created fixed definitions for the astronomical phenomena needed to determine the date of Easter. They used what was called an "ecclesiastic moon" - essentially a virtual moon based on the Babylonian 19-year cycle - and fixed the date of the vernal equinox at March 21 for the purposes of calculation.

The Eastern Churches do NOT follow these definitions. They determine Easter from the real full moon at the longitude of Jerusalem. Therefore, there is often a difference in the date of Easter between the Eastern and Western Churches.

The following is a table listing the date of Easter until 2020:

For those interested in number-crunching and mathematical nuances, the following is an algorithm to calculate the date of Easter:

   Divide                      by       Quotient     Remainder

the year x                     19          -             a
the year x                    100          b             c
b                               4          d             e
b + 8                          25          f             -
b - f + 1                       3          g             -
19*a + b - d - g + 15          30          -             h
c                               4          i             k
32 + 2*e + 2*i - h - k          7          -             L
a + 11*h + 22*L               451          m             -
h + L - 7*m + 114              31          n             p

Then n is the number of the month (3 = March; 4 = April) and p + 1 is the day of that month upon which Easter Sunday falls. The symbol * means the product of the two numbers.

Sources: Astronomical Algorithms, Jean Meeus; Man and the Stars, Robert Hanbury Brown.