This picture taken early on April 8, 2020 shows the closest supermoon to the Earth, also known as a … [+] pink moon, behind the cross on a church in downtown Moscow. (Photo by Kirill KUDRYAVTSEV / AFP) (Photo by KIRILL KUDRYAVTSEV/AFP via Getty Images)

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The moon’s phases determine Easter’s date. Western Easter is the first Sunday after the first full moon after the equinox, while Eastern Easter is after Passover.

This year, Easter—a Christian celebration of Jesus Christ rising from the dead—will be held on Sunday, March 31, for Western Christians and on Sunday, May 5, for Eastern Orthodox Christians.

Why the dates of Easter change every year—and why they differ so much between Western and Eastern Christianity—is primarily a story about astronomy and, more specifically, the moon’s phases.

Easter Is A Lunar Festival

While some religions use a predominantly lunar calendar, Christianity does not, and yet Easter is a lunar festival, just like Ramadan, Passover, and the Chinese Lunar New Year.

At the core of all of this is the lunar year. While Earth takes 365.25 days to orbit the Sun, the moon takes 29.5 days to orbit the Earth. So, a lunar year is 354.3 days. There’s a 10 or 11-day lag between the two—and that explains why the dates of Easter differ so much from year to year.

How The Date Of Western Easter Is Decided

Christians in the West celebrate Easter on the first Sunday after the first full moon occurring on or after the vernal equinox, as determined by the Gregorian calendar.

• This year, the vernal, or spring, equinox occurs on Wednesday, March 20.
• The first full moon after that is the “Worm Moon” on Monday, March 25, which is thus called the “Paschal Moon.”
• The following Sunday is March 31.

Easter is early because a full moon occurs just a few days after the spring equinox, which is more-or-less a fixed date (occurring between March 20-22 each year).

How The Date Of Eastern Easter Is Decided

Easter is celebrated by Christians in the east on the first Sunday after Sunday after the first full moon after the Jewish festival of Passover (Pesach). Passover celebrates the Biblical story of the Israelites’ escape from slavery in Egypt.

Passover always begins on the 15th day of the Hebrew month of Nisan. Since the Hebrew calendar is largely lunar-based, Passover is celebrated in 2024 on Monday, April 22. That’s the date of the fourth full moon of the year—the “Pink Moon.”

The Sunday after Passover is Sunday, May 5.

Why Both Easters Are Often The Same Date

Suppose a full moon occurs just before the date of the spring equinox. In that case, Western Christianity’s “Paschal Moon” won’t happen until mid-to-late April, thus delaying Easter Sunday and making it identical to the date of Eastern Orthodox Christianity’s Easter. That last happened in 2017 and will happen again in 2025.

Wishing you clear skies and wide eyes.