It’s not often that Easter Sunday falls so late in the year — nearly a week after Tax Day. Easter and related Christian holidays — Ash Wednesday, Palm Sunday and Good Friday, are called moveable feast days because they occur on different dates every year. The Jewish holiday of Passover also changes every year.

This year’s April 21 holiday is the latest Easter since 2000 and it will be the latest until April 25, 2038.

So, why is Easter so late this year? The date is based on rules set by the Council of Nicaea in 325 A.D. The rules depend partly on the phase of the moon, but other complicated mathematical rules also come into play.

Easter Sunday is held after the first full moon of spring, also called the Passover Full Moon or the Paschal Full Moon. It can come as early as March 22 and as late as April 25.

This year, the first day of spring was on Wednesday, March 20, while the Paschal Full Moon was on March 21. So, it should follow that Easter would have come on March 24, but it didn’t. Space.com explains the “clerical loopholes” that account for this. For the purpose of calculating the dates of Easter, the full moon is reckoned according to an ecclesiastical computation and the “ecclesiastical” full moon does not necessarily coincide with the real, “astronomical” full moon. Likewise, the equinox is always assumed to fall on March 21 in the church calculations.

Following all of the rules, so far as the Christian church is concerned, Easter was on hold until after the April 19 full moon.

Next year the trend will begin to reverse itself, with Easter 2020 falling on April 12; it will also be observed on April 4, 2021; April 17, 2022; and on March 31, 2024. A fun fact: Easter Sunday cannot occur in March for two successive years. The most common date is April 19.