How Is the Date of Easter Determined?

Easter Sunday is on a different date each year, but how exactly is it calculated?

The rules date back to the year 325, and the First Council of Nicaea. Christian bishops gathered to agree on various aspects of their church, and among other things, they decided on how Easter would be calculated.

Although the calculation is a little obscure at first, but it's actually quite simple in practice. The short version is this: It's the first Sunday after the first full moon that occurs on, or after, March 21st. For example, in 2013, the first full moon after March 21st is on the 27th (a Wednesday), so Easter Sunday follows a few days later, on the 31st.

A few more points may help to clarify:

A full moon that occurs on March 21st is valid for the computation.

Easter must come after the full moon, so if the full moon happens to be on a Sunday, then Easter is the following Sunday.

That means the earliest possible date for Easter is March 22nd, and this can only happen when the full moon rises on Saturday, March 21st.

About twice a year, day and night are roughly of equal length because of the tilt of the Earth towards the Sun. This is called an equinox, and in spring (in the Northern Hemisphere), it's around March 20th. It varies from year to year, however, and can be as early as March 19th, or as late as March 21st.

At the First Council of Nicaea in 325 AD, the Christian bishops came together to try and set some things straight in the church. One of the big issues was to determine a date for Easter, which up to then, relied on the Jewish calendar, particularly the feast of the Passover.

The Synoptic Gospels actually describe the Last Supper as a Passover meal. However, some believed that relying on the Jewish calendar for this date was unreliable, as sometimes a month was being chosen that predated the March equinox.

To overcome these difficulties, they decided on their own independent computation of the date. The starting point was the spring equinox, but they incorrectly identified it as being fixed on March 21st.

Over the centuries, the inaccuracies in the calendar pushed the real equinox earlier and earlier, until Pope Gregory XIII introduced a new system, including leap years, which became known as the Gregorian calendar that we use today. This kept the spring equinox date consistent over time. With a fixed date for the March equinox in place, calculating Easter became far more predictable, and this system remains with us today