How Is the Date of Easter Determined?

A Simple Formula Determines the Date of Easter Each Year

Easter celebrates the Resurrection of Christ
The Resurrection of Jesus Christ. Fine Art Images/Heritage Images/Getty Images

Easter, the Christian holiday that celebrates the day of the resurrection of Jesus Christ, is a moveable feast, which means that it does not occur on the same date every year. Easter is calculated based on the phases of the moon and the coming of spring.

Determining the Date of Easter

In 325 A.D., the Council of Nicaea, which agreed upon the basic principles of Christianity, established a formula for the date of Easter as the Sunday following the paschal full moon, which is the full moon that falls on or after the spring equinox. In practice, that means that Easter is always the first Sunday after the first full moon that falls on or after March 21. Easter can occur as early as March 22 and as late as April 25, depending on when the paschal full moon occurs.

You can easily find the date of Easter in this and future years, in both the Western (Gregorian) and Eastern (Julian) calculations online.

The Significance of the Paschal Full Moon

The Council of Nicaea decided that Easter must always occur on a Sunday because Sunday was the day on which Christ rose from the dead. But why is the paschal full moon used to determine the date of Easter? The answer comes from the Jewish calendar. The Aramaic word "paschal" means "pass over," which is a reference to the Jewish holiday.

Passover fell on the date of the paschal full moon in the Jewish calendar. Jesus Christ was Jewish. His  Last Supper with his disciples was a Passover Seder. It is now called Holy Thursday by Christians and is the Thursday immediately before Easter Sunday. Therefore, the very first Easter Sunday was the Sunday after Passover.

Many Christians erroneously believe that the date of Easter is currently determined by the date of Passover, and so they are surprised when Western Christians sometimes celebrated Easter before the Jewish celebration of Passover.

Approximate Dates for the Paschal Moon

The paschal full moon can fall on different days in different time zones, which can present a problem when calculating the date of Easter. If people in different time zones were to calculate the date of Easter depending on when they observed the paschal full moon, then that would mean that the date of Easter would be different depending on which time zone they lived in. For that reason, the church does not use the exact date of the paschal full moon but an approximation.

For calculation purposes, the full moon is always set on the 14th day of the lunar month. The lunar month begins with the new moon. For the same reason, the church sets the date of the spring equinox at March 21, even though the actual vernal equinox can occur on March 20. These two approximations allow the church to set a universal date for Easter, regardless of when you observe the paschal full moon in your time zone.

Occasional Different Date for Eastern Orthodox Christians

Easter is not always celebrated universally by all Christians on the same date. Western Christians, including the Roman Catholic church and Protestant denominations, calculate the date of Easter by using the Gregorian calendar, which is a more astronomically precise calendar that's used throughout the West today in both the secular and religious worlds.

Eastern Orthodox Christians, such as the Greek and Russian Orthodox Christians, continue to use the older Julian calendar to calculate the date of Easter. The Orthodox Church uses the exact same formula established by the Council of Nicaea for determining the date of Easter only with a different calendar.

Because of the date differences on the Julian calendar, the Eastern Orthodox celebration of Easter always occurs after the Jewish celebration of Passover. Erroneously, Orthodox believers may think their Easter date is tied to Passover, but it isn't. As the Antiochian Orthodox Christian Archdiocese of North America explained in a 1994 article entitled "The Date of Pascha."

A Theological Controversy

The Council of Nicaea set up a formula for calculating the date of Easter to separate the Christian celebration of Christ's Resurrection from the Jewish celebration of Passover.

While Easter and Passover were related historically—the Council of Nicaea ruled that because Christ is symbolically the sacrificial Passover lamb, the holiday of Passover no longer has theological significance for Christians.