Easter is a celebration of Jesus' Resurrection from the dead on the Sunday morning following his crucifixion. From the earliest centuries of Christianity, believers have remembered Jesus' Resurrection at weekly Sunday gatherings and in an annual celebration of Easter.
The Gospels record that Jesus was crucified at the time of the Jewish Passover feast. The Jewish calender was a lunar calender, in which each month begins and ends with a new moon. Passover was always in the first lunar month, which fell in the spring on the Jewish calender. It was always on the 14th day of the month, when the moon was full.
When Christians began celebrating the Resurrection of Jesus, they tried to stay close to the date of the Passover feast, even though they were using a calender that was based on the movements of the sun and not the moon. After considerable controversy about the correct way to set the date of Easter, the churches in the early fourth century agreed to a relatively simple formula: Easter would be observed on the first Sunday on or after the first full moon on or after the spring equinox, which always falls on March 21.
