Have you ever wondered why Easter Sunday changes each year? We have other holidays that shift annually – Thanksgiving, for example, is always on the fourth Thursday of November. That means it can occur on any date from November 22 to 28.

Mother’s Day also wanders around the calendar, as does Father’s Day, Labor Day, Memorial Day and President’s Day.

However none of them are as unpredictable as Easter.

“At the heart of the matter lies a very simple explanation,” writes Mary Fairchild on the website Ask.com. “The early church fathers wished to keep the observance of Easter in correlation to the Jewish Passover.”

After all, the death and burial of Jesus Christ occurred on the day before Passover. Christ’s resurrection happened the day after Passover.

So, the church “wanted Easter to always be celebrated subsequent to the Passover,” explains Fairchild.

However, the date for Passover changes every year. “The Jewish holiday calendar is based on solar and lunar cycles. Each feast day is movable, with dates shifting from year to year,” notes Fairchild.

So, what could have been simple got immediately complicated.

If the church had stuck to the original plan, Easter would always be celebrated at Passover.

However, the church and the Jewish community have not always gotten along. The Spanish Inquisition, for example, was not exactly a model of religious unity or a celebration of diversity. The bad relations climaxed, one might say, with Hitler’s attempt to wipe Jews off the face of the earth.

However, Jesus was Jewish and taught love and forgiveness. So, the church’s contradiction of loving everybody – except Jews – stems from the New Testament account of Jesus’ trial in Matthew 27:24–25, "When Pilate saw that he was getting nowhere, but that instead an uproar was starting, he took water and washed his hands in front of the crowd. 'I am innocent of this man’s blood,' he said. 'It is your responsibility!' All the people answered, 'His blood is on us and on our children!'"

As a result, 2,000 years, Jews were regarded as “Christ killers.” Only in the 1960s was the taint officially lifted by Christianity’s largest group, the Roman Catholics. As a part of the Second Vatican Council (1962–1965), Pope Paul VI issued a declaration which repudiated collective Jewish guilt for the crucifixion of Jesus.

In other words, today’s Jews cannot be held responsible for Christ’s crucifixion. They weren’t there. They had nothing to do with it. They weren’t born yet.

However, in the third century, resentment was still raw. And the church didn’t like having to ask the Jews each year when Easter should be celebrated.

So, in the year 325 A.D., the Council of Nicaea “set the date of Easter as the Sunday following the paschal full moon,” writes Scott P. Richert on the About website.

What is paschal full moon? That’s the first full moon either on or after the spring equinox.

What is the spring equinox? An equinox occurs twice a year, around March 20 and September 22. You’d think that the equinox is the day when daytime and night are of the same length. Sure, the word equinox comes from this definition, derived from the Latin aequus (equal) and nox (night). However, the equinox is not exactly the same as the day when period of daytime and night are of equal length.

Why not? Sunrise, which begins daytime, occurs when the top of the sun's disk rises above the eastern horizon. At that instant, the disk's center is still below the horizon. Second, Earth's atmosphere refracts sunlight. As a result, an observer sees daylight before the first glimpse of the sun's disk above the horizon.

So, if we want to be proper, we should use the word equilux to describe the 24-hour period in which the periods of daylight and night are equal. Also, the times of sunset and sunrise vary with an observer's longitude and latitude, so the date when day and night are of exactly equal length depends on your location.

Basically, the equinox occurs on different days in different locations around the globe.

So, it’s not an accurate way in which to base anything – particularly when to celebrate Easter.

Prior to 325 A.D., Easter was always celebrated on the Sunday immediately following the paschal full moon. However, after the Council of Nicea, it was decided to establish a more standardized system for determining the date of Easter. As a result, explains Fairchild, today “the date of the paschal full moon is determined from historical tables, and has no correspondence to lunar events.”

“Astronomers were able to approximate the dates of all the full moons in future years. The western Christian church used these calculations to establish a table of ‘Ecclesiastical Full Moon’ dates. These dates would determine the Holy Days on the Ecclesiastical calendar.”

However, that didn’t work very well either.

The solution should have been simple – just go back to celebrating Easter whenever the Jewish community was celebrating Passover. However, that was politically incorrect.

So, around 1583 A.D., the table for determining the Ecclesiastical Full Moon dates was permanently established and has been used ever since to determine the date of Easter. As a result, it’s rare that Easter falls on Passover.

Easter has ever since then been celebrated on the Sunday immediately following the paschal full moon. However, the paschal full moon can vary as much as two days from the date of the actual full moon, with dates ranging from March 21 to April 18.

So, Easter dates can wildly swing from March 22 through April 25.

The bottom line? Forget about trying to calculate next year’s date for Easter by yourself. Just look in your Farmer’s Almanac or Google it or glance up at the calendar on the wall.

And if you didn’t think the matter could get any worse, it does. There are two Easters celebrated annually around the world.

When the Roman Empire split in two almost 2,000 years ago – with the western empire centered in Italy and the eastern empire headquartered in Turkey, the church split as well.

There were a lot of theological reasons, such as the wording of the official creed and Mary’s role. However, today, the two groups can’t agree about much of anything beyond “Jesus is Lord.”

And they celebrate Easter on different dates.