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Why does Easter change every year? Here's how the date has been historically determined.

USA TODAY

Correction: A previous version of this article mentioned the 2023 date for the Eastern Orthodox Easter instead of the current鈥檚 year. The date has been updated.

Easter, a Christian holiday that celebrates the resurrection of Jesus聽Christ, occurs on a Sunday in spring every year. This聽makes聽it a movable feast because its actual date changes.

This year, Easter falls on March 31. The next time it will occur on March 31 will be 2086.

The simplest way to explain the date of Easter is that it falls on the first Sunday after the full moon that follows the spring equinox.

Because the actual date of the spring equinox can differ by a day or two, the Catholic Church created a fixed date of March 21 to define it, known as .

And the full moon that occurs just on or after March 21 is known as the Paschal full moon. Paschal derives from "pascha," the Greek and Latin word for "passover."

The Paschal full moon appeared on March 25 this year. So Easter is on March 31, the first Sunday after the full moon's appearance.

When can Easter occur?

The U.S. and most of the world use the Gregorian calendar, which can place the date of Easter anywhere between March 22 and April 25.

The Gregorian calendar, instituted by Pope Gregory XIII, .

In the 1,417 years from 1583 through 3000, it鈥檚 most common for Easter to fall on April 16, with 61 instances. The least common dates are March 22 and 24, with nine instances apiece.

Eastern Orthodox churches , which can place the date of Easter anywhere between April 4 and May 8聽when it's converted back to the Gregorian calendar. This year, the Eastern Orthodox Church observes Easter on Sunday, May 5.

How was it calculated before?

Calculating the date of Easter has been a complex matter for thousands of years because of the intricacies of accounting for both the solar and lunar cycles, as well as the Gregorian and Julian calendars.

As early as the year 222, Romans were using different methods to聽calculate聽the dates of Easter. Some聽methods relied on the .

Churches still rely on earlier calculations of the moon phases as well as its set definition of the spring equinox, which can聽place Easter on .