Unlike Christmas, which falls on the same date every year, Easter is what’s called a “moveable feast." That means that Easter falls on a different date from year to year, moving throughout the calendar. In 2019, Easter falls on April 21 — a full three weeks later than Easter in 2018, which fell on April 1. The way that Easter moves throughout the calendar has a surprising factor: the moon.
To understand way, we have to go back to the roots of Easter. The Bible doesn’t spell out the exact date that Easter occurs on, but it does say that Jesus was crucified during the Jewish holiday of Passover. According to the Catholic magazine America: The Jesuit Review, in the year 325, the Council of Nicaea decided to celebrate Easter “at the very time of Jesus’ Passion," the Christian term for the final days of Jesus' life before his death and resurrection. The Jewish calendar is calculated based on lunar months, so to link Easter with Passover, the Council of Nicaea decided that Easter would be observed “on the first Sunday after the first full moon on or after the vernal equinox.” Basically, because the moon affects when Passover falls, the moon also affects when Easter falls.
Back in 1582, Pope Gregory XIII made some additional rules about calculating Easter's date when introducing the Gregorian calendar. According to Space.com, these rules said that Easter would always fall between March 22 and April 25 — and that the Catholic Church would always mark the vernal equinox on March 21, even though the astronomical vernal equinox can be anywhere from March 19 to March 21. This year, the first full moon after March 21 was on April 19, which is a Friday — meaning that this Easter will be on the following Sunday, April 21. (You don’t have to check your calendar — that’s correct.)
But while the “vernal equinox = March 21” rule explains why Easter is on April 21 this year, there are some additional rules that have made calculating Easter even more complicated in other years. Space.com points out that the “astronomical” full moon and “ecclesiastical” full moon do not always line up, which affected the date of Easter back in 1974. Additionally, most (but not all) Eastern Orthodox churches celebrate Easter up to five weeks later than Catholic and Protestant churches. That’s because, according to the BBC, they use the Julian calendar to calculate the date for Easter and some other holy days.
Throughout history, there have been people who wanted to do away with all these calculations and make Easter a fixed date for all Christians — and some still hold this goal. In 2016, the Archbishop of Canterbury, the Most Rev. Justin Welby, the head of the Anglican church, announced that Anglican leaders would join Orthodox and Catholic leaders to discuss setting a fixed date for Easter, reported the Daily Telegraph. Archbishop Welby said that this date would most likely the second or third Sunday in April, and would likely be decided “between five and 10 years' time.”
"I think the first attempt to do this was in the 10th century,” he joked.