Ecclesiastical Calendar:

smart.net

This program calculates:

Easter Sunday in both the Western (Catholic, Anglican, and Protestant) Christian, and in the Eastern (Orthodox) Christian traditions;

The movable Feasts associated with Easter in the Western and Eastern traditions;

Many Feasts in the Catholic calendar, usually celebrations of events in the lives of Jesus or Mary. (These Feasts may or may not be celebrated by other Christian denominations.)

For years before 1583, only the dates of Christmas and Easter are calculated. (They are the same for both traditions pre-1583.) My recent research (email and bookwork) has shown that the above method for calculating Easter prior to AD1583 was not in wide usage until the mid-eighth century, although it had been adopted by the Church in the mid-sixth century.

An important note for historians and people using these dates for research: Even though the Gregorian calendar was adopted into use by the Catholic Church and many Catholic areas of Europe in AD 1582 October, many areas did not adopt the Gregorian Calendar, the new method of determining Easter, or both, until later. A good review is given in reference (1). For example, England and its dominions did not accept the Gregorian Calendar or the new method of determining Easter until 1752; thus, Easter in England prior to 1753 was determined using the same algorithm as that of the Orthodox Church.