The date when Easter will be celebrated was decided upon at the First Council of Nicea, a council of Christian bishops convened in Nicea by the Roman Emperor Constantine I in 325AD. Nicea was on the site of the modern-day town of Iznik in Turkey.
The purpose of the gathering was to make an agreement on several Christian topics, including the date of Easter.
The bishops decided that Easter Day (also called Easter Sunday, the anniversary of Christ’s resurrection) would fall on the first Sunday following the paschal full moon. That means the next full moon after the spring equinox.
Part of the thinking behind a flexible date is that Easter must always occur on a Sunday, because that was the day of Christ’s Resurrection.
The reason for choosing the paschal full moon is that it’s the date of Passover in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, Easter is the Sunday after Passover.
However, the paschal full moon can be on different days in different time zones and that would produce variations on Easter.
Realising this, the Church decided that the full moon is always determined to be the 14th day of the lunar month. The Church also fixes the spring equinox as March 21 – even though it can occur on March 20, as it will in 2014 – so that it can help to work out the date of Easter.