In Western Christianity, Easter is celebrated on the Sunday following the Paschal Full Moon date. The Paschal Full Moon date is determined from historical tables and doesn't always correspond with the astronomical full moon. Instead, the Paschal Full Moon date is the first ecclesiastical full moon date after March 20.
Western Churches use the Gregorian calendar for date calculations. Eastern Orthodox churches generally use the Julian calendar, further complicating matters. Additionally, many Eastern churches also use the proper astronomical full moon and vernal equinox, as observed from the meridian of Jerusalem, to calculate the date for Easter.
Early Christians believed that the death, burial and resurrection of Jesus Christ happened after the Jewish Passover festival, and kept their observance of the occasion in line with the Jewish holiday calendar. Unlike the sun-based Julian and Gregorian calendars, the Jewish holiday calendar is based on the movement of both the sun and the moon, leading to shifting dates. Easter dates varied among early Christians. The Council of Nicea, in 325 A.D. started the process of holiday date standardization.
Some groups have made attempts to reform Easter dating calculations. In the United Kingdom, the Easter Act of 1928 attempted to place Easter on the second Saturday in April. The 1997 the World Council of Churches proposed a mathematical calculation based on direct astronomical observation. These reforms remain unimplemented.