How is the date of Easter determined?

Written on April 25, 2011

The Easter holiday occupies the center of the ecclesiastical calendar and is used to determine the feast days in the liturgical cycle. The choice of its date is therefore especially important, in terms of both civil and religious life. The Easter holiday varies between March 22 and April 25, which corresponds to 35 different dates. But how is that date chosen?

A complex liturgical calendar

Beginning in the fourth century, the Roman Catholic Church adapted the Julian calendar and gradually “Christianized” it. In other words, little by little the Church introduced religious holidays that drew attention to the main events in the life of Jesus Christ (his birth, his death, his resurrection) and in the life of the Virgin Mary. The Christian Church’s liturgical calendar is complex, since it draws on both a lunar calendar and a solar calendar. Religious holidays are divided into two groups:

• movable feasts, of which the chief one is the Easter holiday, and which are linked to the lunar calendar; and
• immovable feasts, which on the contrary are dependent on the solar calendar, with Christmas being the most significant.

The choice of the date for Easter, which occupies the center of the ecclesiastical calendar, will determine the calendar of holidays in the liturgical cycle, the moveable feasts in particular. Determining the date of the Easter holiday is therefore extremely important for the Church.

The date for Easter, a cycle of 57,000 centuries

After some centuries of confusion on the subject, the Council of Nicaea in the year 325 finally set the rule for determining the date for Easter (unchanged ever since): “The Easter holiday is celebrated on the Sunday that follows the fourteenth day of the Moon when it reaches that age on March 21 or immediately afterwards.” The Easter date can thus fall anywhere between March 22 and April 25, a range of 35 possible dates.

An uneven distribution of Easter dates

Should the Moon reach its 14th day on March 21, Easter will be celebrated on March 22 if that day is a Sunday. If the Moon reaches its 14th day on March 20, the Easter Moon will fall on April 18 (lunation of 29.53 days). Should that date be a Sunday, Easter will take place on April 25. At first sight, with the full Moon potentially falling on any date at all, you’d think there would be an even distribution of Easter dates. But that’s not the case. Over the cycle of 57,000 centuries, April 19 turns out to be favored because of the two exceptions in the calculation of the Easter date. Thanks to those, statistically we get a 3.87 percent chance of landing on the date of April 19. For the period stretching from 1583 to 2539, the final possible date, April 25, appears only nine times. The last time Easter fell on April 25 was 1943. And the next occasion will be in 2038!

Date of Easter Sunday  2021-2030