What determines the date for Easter Sunday? by Martin Galea De Giovanni & Alexei Pace

Christians celebrate two major feasts that symbolise the birth and death of Christ. Unlike Christmas, which as we all know is always on the 25th of December, Easter Sunday does not fall on a same day every year. The date of Easter is primarily used for liturgical purposes. Up to the eighth century there was no uniform method for determining the date of Easter but the method favoured by the Council of Nicaea in AD 325 gradually became the accepted method. The adoption of the Gregorian calendar necessitated some modifications to this scheme but it is still basically the same

The "popular" rule (for Roman Catholics and most Protestant denominations) is that Easter is on the first Sunday after the first full moon after the March equinox. The actual rule is similar, except that the astronomical equinox is not used; the date is fixed at March 21. And the astronomical full moon is not used; an "ecclesiastical" new moon is determined by adopted tables based on the Metonic cycle, and "full" is taken as the 14th day of that lunation. There are auxiliary rules that make March 22 the earliest possible date for Easter and April 25 the latest. The reasons for this are that the method is then independent of longitude on the Earth and is thus independent of Time Zone. It also allows the date of Easter to be calculated in advance regardless of the actual motion of the Earth around the Sun.

This also removed the complications which arose whenever the year was a leap year. The calendar year is 365 days long, unless the year is exactly divisible by 4, in which case an extra day is added to February to make the year 366 days long. If the year is the last year of a century, for example 1700, 1800, 1900, 2000, then it is only a leap year if it is exactly divisible by 400. Therefore, 1900 wasn't a leap year but 2004 was. The reason for these rules is to bring the average length of the calendar year into line with the length of the Earth's orbit around the Sun, so that the seasons always occur during the same months each year.

In 2008 we have an "early" Easter. This is because the last full moon before the spring equinox is on February 21. The spring equinox occurs on March 20. The first full moon after the equinox will come on Friday, March 21 at 6:40 pm (Universal Time). Easter Sunday therefore is March 23, 2008.