How Is the Date of Easter Determined?

A Simple Formula Determines the Date of Easter Each Year

Resurrection of Christ
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Easter is a moveable feast, which means that it does not occur on the same date every year. How is the date of Easter calculated?

Here's the Formula for Determining the Date of Easter

In A.D. 325, the Council of Nicaea set the date of Easter as the Sunday following the paschal full moon, which is the full moon that falls on or after the vernal (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.

What Is 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. Passover fell on the date of the paschal full moon in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, the very first Easter Sunday—the day on which Christ rose from the dead—was the Sunday after Passover.

For Calculation Purposes, Approximate Dates Are Necessary

The paschal full moon can fall on different days in different time zones, which presents 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, 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 at 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 vernal 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.

Why Do the Eastern Orthodox Celebrate Easter on a Different Date?

Still, Easter isn't celebrated universally on that date—at least not on the calendar we all use in everyday life. While Western Christians calculate the date of Easter by using the Gregorian calendar (the calendar that's used throughout the West today, in both the secular and religious worlds), the Eastern Orthodox continue to use the older, astronomically inaccurate Julian calendar to calculate the date of Easter.

It's important to note that the Orthodox use the exact same formula for determining the date of Easter—the formula that I gave above, established by the Council of Nicaea, in which Easter is the Sunday following the paschal full moon, which is the full moon that falls on or after the vernal (spring) equinox.

Currently, on the Julian calendar, March 21 (the date of the vernal equinox) falls on April 3 on the Gregorian calendar. Therefore, for the Orthodox, the Sunday following the paschal full moon has to fall after April 3 on the Gregorian calendar. This has the effect, in most years, of pushing the date of Orthodox Easter later than the date of Western Easter on the Gregorian calendar. Again, though, the Orthodox are using the exact same formula to determine the date of Easter; the entire difference in the date comes from using the Julian calendar to calculate the date rather than the Gregorian one.

Is the Date of Easter Related to Passover?

Many Christians believe that the date of Easter is determined by the date of Passover, and so they are surprised when, in years such as 2008, Western Christians celebrate Easter before the Jewish celebration of Passover. (See Reader Question: Why Does Easter Come Before Passover This Year?)

But look closely once again at the formula for calculating Easter, established at the Council of Nicaea: Easter is celebrated on the Sunday following the paschal full moon. Notice what isn't mentioned? That's right: Passover.

A Theological Controversy

In fact, the reason the Council of Nicaea set up a formula for calculating the date of Easter was to separate the Christian celebration of Christ's Resurrection from the Jewish celebration of Passover. Some Christians in the early fourth century were calculating the date of Easter according to the Jewish calendar, while others believed it needed to be calculated separately.

The Council of Nicaea agreed with the latter group. While Easter and Passover were related historically—as I noted above, the Last Supper occurred on the Passover—and theologically, the Council declared that the connection between the two holidays ended with the Death and Resurrection of Christ.

Why? Because Christ is the new Paschal Lamb: The lamb sacrificed at Passover before Christ's Death and Resurrection was just a sign or symbol of Christ Himself. That symbol found its completion in the sacrifice of Christ on the Cross. Thus, the continued celebration of the Jewish Passover has no theological significance for Christians, because Christ is our Passover Lamb.

That is why Easter can, and often does, fall before Passover today.

But Don't the Eastern Orthodox Calculate Easter With Reference to Passover?

Still, many Christians remain confused on this point, because currently, the Eastern Orthodox celebration of Easter always occurs after the Jewish celebration of Passover. Therefore, many people assume that the Eastern Orthodox calculate the date of Easter differently and that the Orthodox Church includes in its calculation a consideration of the modern date of Passover.

But as I mentioned several times above, the Orthodox use the very same formula established by the Council of Nicaea. As the Antiochian Orthodox Christian Archdiocese of North America explained in a 1994 article entitled "The Date of Pascha," the modern date of Passover has nothing to do with the Orthodox date of Easter.

The difference between the Western date of Easter and the Orthodox date of Easter is, as the article explains, entirely the result of the Orthodox Church's continued use of the astronomically inaccurate Julian calendar. As noted above, on the Julian calendar March 21—the ecclesiastical date of the vernal equinox—falls on April 3 on the Gregorian calendar, which means that Easter, for the Orthodox, can fall no earlier than April 4 on the Gregorian calendar. (For more details, see Is the Date of Easter Related to Passover?)

Looking for the Date of Easter?

You can find the date of Easter in this and future years, in both the Western (Gregorian) and Eastern (Julian) calculations, in When Is Easter?

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Richert, Scott P. "How Is the Date of Easter Determined?" ThoughtCo, Apr. 5, 2017, Richert, Scott P. (2017, April 5). How Is the Date of Easter Determined? Retrieved from Richert, Scott P. "How Is the Date of Easter Determined?" ThoughtCo. (accessed February 21, 2018).