This year, Easter is March 31.The holiday can be anytime from two days to 31 days after the first full moon of spring.
Gene Davenport said the date for Easter is based on the lunar calendar.
Davenport is chairman of the religion and philosophy department at Lambuth University.
Easter is tied to the Jewish holiday of Passover to some extent, he said. "Passover is always the first full moon after the first day of spring. Easter is always the first Sunday after that first full moon."
Jesus was crucified during Passover, on Good Friday.
Passover is a Jewish festival that commemorates the Exodus, which was the liberation of the Israelites from Egyptian slavery.
One question that arises when some look at the Gospels is: Why not have Easter three days after Passover, since Christ rose in three days?
"I think the emphasis is on Sunday, which is the first day of the week," Davenport said. "The resurrection happened on a Sunday. Some churches consider every Sunday Resurrection Sunday."
There was debate in the fourth century over whether the church should have Easter on a fixed date.
"But the Passover tie was really the decisive element," he said.
Several other holy days are calculated on the basis of Easter, such as Lent, which is 40 days before Easter.
"Shrove Tuesday is the last day of Epiphany," he said. Epiphany is 12 days after Christmas and celebrates the arrival of the Magi. "The later Easter comes, the longer Epiphany is."
The date for Easter can vary by six weeks, Davenport said.
"If the full moon comes the day before the first day of spring, you have to wait until the next full moon for Passover."
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, he said.
Nettles said 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, called the Nicene Creed. The council was created to settle theological disagreements.
"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," he said. "The spring equinox will happen on either March 20 or 21. The dates for Easter float between March 22 and April 25."
There was a controversy around 190 A.D. about when to celebrate the Easter holiday.
"It was between the people who celebrated Easter on Passover, which is full moon day in the Jewish month of Nisan," he said.
Other Christian groups believed Easter needed to always be on Sunday, because Christ rose from the grave on Sunday, Nettles said.
"They had a meeting to decide this," he said. "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."
This means sometimes Easter Sunday is a month before Passover, and sometimes Easter is on or near Passover, Nettles said.
"That happens because Passover depends on the lunar calendar," he said. "And Easter is tied very closely to a solar date - the equinox. We have to take into account the lunar date of the full moon at the same time."
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, which places Easter 13 days later.
"The orthodox churches didn't agree with Pope Gregory, who changed the calendar to what we know today," Nettles said.