In the early days of the Christian church, Easter was celebrated on the Sunday immediately following the first astronomical full moon after the vernal (spring) equinox. Over the course of history, beginning in 325 AD with the Council of Nicea, the Western Church decided to established a more standardized system for determining the date of Easter. Astronomers were able to approximate the dates of all the full moons in future years for the Western Christian churches, thus establishing a table of Ecclesiastical Full Moon dates.

These dates would determine the Holy Days on the Ecclesiastical Calendar.

Though modified slightly from its original form, by 1583 AD the table for determining the Ecclesiastical Full Moon dates was permanently established and has been used ever since to determine the date of Easter. Thus, according to the Ecclesiastical tables, the Paschal (Passover) Full Moon is the first Ecclesiastical Full Moon date after March 20 (which happened to be the vernal equinox date in 325 AD). So, in Western Christianity, Easter is always celebrated on the Sunday immediately following the Paschal Full Moon.

The Paschal Full Moon can vary as much as two days from the date of the actual full moon, with dates ranging from March 21 to April 18. As a result, Easter dates can range from March 22 through April 25 in Western Christianity.

For more about Easter dates, Paschal Full Moon and Ecclesiastical tables visit:
• Why Does the Date for Easter Change Every Year?
• Easter Dating Method
• Christian History article by Farrell Brown
• Easter Dating
• The Calendar of the Orthodox Church