Easter and Other Dates in the Liturgical Calendar

The date of Easter was defined during the reign of Constantine by the Council of Nicæa in the year 325 as the first Sunday after the first full moon which falls on or after the Vernal Equinox. The Equinox is assumed to always fall on March 21st, so the calculation requires a determination of the date of the full moon and the date of the following Sunday.

Under the Julian Calendar for years before 1753 a simple 19-year cycle is used to track the phases of the Moon.

Under the Gregorian Calendar (for years after 1753 - devised by Clavius and Lilius, and introduced by Pope Gregory XIII in October 1582, and into Britain and its then colonies in September 1752) two correction factors were added to make the cycle more accurate.

The algorithm used here was introduced around the year 532 by Dionysius Exiguus. Our thanks to the U. S. Naval Observatory for putting it into computer code.