The date of Easter changes yearly because it is a movable feast that the Christian church calculates to be on the first Sunday after the full moon, known as the Paschal full moon, following the vernal equinox. The equinox is fixed as March 21, so depending on the appearance of the full moon, Easter can fall anywhere between March 22 and April 25.
The Early Church
In the early church, the date of Easter was linked to the Jewish feast of Passover. Because Christians believed that Jesus's death and resurrection occurred after Passover, they wished to observe the holiday after the Jewish feast each year. This meant that Christians were dependent on Jewish experts to calculate the exact date of Easter each year.
At the First Council of Nicaea in 325 A.D., Christian bishops decided to separate Easter from the Jewish calendar, in order to determine the date in a simpler way. Although the equinox often occurs on March 20, for uniformity it was set on March 21. Astronomers created a table for calculating the approximate dates of Paschal full moons with which they were able to determine the yearly dates of Easter.
Date Discrepancies in Eastern and Western Churches
The Western Christian church does not always celebrate Easter on the same day as the Eastern Orthodox church because the Western church uses the Gregorian calendar, while the Eastern Orthodox church uses the Julian calendar. Because of the 13-day difference between the two calendars, in Eastern Orthodox churches, Easter falls between the Gregorian calendar dates of April 4 and May 8.
In addition to Eastern Orthodox Churches using the Julian calendar to calculate the date of Easter, they also use the true astronomical full moon and vernal equinox as observed in Jerusalem.