The holiday can be anytime from two to 31 days after the first full moon of spring.
For the years 1875 through 2124, there are only two occurrences of a March 23 Easter. The first one was in 1913, and this year will be the second one.
Easter is tied to the Jewish holiday of Passover.
This dating of Easter is based on the lunar calendar that Easter is always the first Sunday after the first full moon after the Spring Equinox (which is March 20)
Passover is a Jewish festival that commemorates the Exodus, which was the liberation of the Israelites from Egyptian slavery.
"If the full moon comes the day before the first day of spring, you have to wait until the next full moon for Passover." Says Bill Nettles, a physics professor at Union University.
If the date for Easter Sunday comes a day before Passover, Easter is moved to the next Sunday.
Leap years do not affect where Easter is placed on the calendar.
Council of Nicea
Nettles says that in 325 A.D., the Council of Nicea decreed when Easter would be for most of the western Christian world.
The Council of Nicea was the first Ecumenical council of the Catholic Church and resulted in the first uniform Christian doctrine. The council was created to settle theological disagreements. And one of the disagreements was when to celebrate Easter.
According to Nettles, the council was asking: "Which Sunday should it be? The Sunday closest to Passover or on different Sundays? Nicea decided to have it on the first Sunday on the first full moon of spring."
He adds, "Because the days of the full moon are 29 and a half days apart, the times of the first full moon can drift around within a month. The spring equinox will happen on either March 20 or 21. The dates for Easter float between March 22 and April 25."
This means sometimes Easter Sunday is a month before Passover, like this year, and sometimes Easter is on or near Passover, Nettles says.
The Roman Catholic Church and most Protestant churches celebrate Easter according to the Gregorian calendar. The Gregorian calendar was designed to keep the vernal equinox on or close to March 21, so that the date of Easter remains correct with respect to the vernal equinox. The orthodox Christian churches celebrate the holiday according to the old Julian calendar.
"The orthodox churches didn't agree with Pope Gregory, who changed the calendar to what we know today," Nettles says.