Easter is April 5, why does its date change every year?

Easter is the Christian celebration of the resurrection of Jesus Christ, but unlike holidays with a fixed date, this one changes every year.

This year, Easter lands on April 5 — the last time it happened was back in 2015, and we won’t see it again until 2037.

To calculate its date: Easter always falls on the first Sunday after the first full moon following the spring equinox. Essentially, if you want to know when to hunt for eggs and Peeps, you have to keep an eye on the moon!

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 the ecclesiastical equinox.

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 will appear on April 1 this year, no joke. So Easter is on April 5, 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, went into effect in 1582.

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

Eastern Orthodox churches use the Julian calendar, which can place the date of Easter anywhere between April 4 and May 8 when it's converted back to the Gregorian calendar. This year, the Eastern Orthodox Church observes Easter on April 12.

How was Easter 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 calculate the dates of Easter. Some methods relied on the 19-year Metonic cycle of the moon.

Churches still rely on earlier calculations of the moon phases as well as its set definition of the spring equinox, which can place Easter on a date different from what is astronomically accurate.