The most important set of moveable feasts are a fixed number of days before or after Easter Sunday, which varies by 35 days since it depends partly on the phase of the moon and must be computed each year. In Eastern Christianity (including the Eastern Orthodox Church, the Oriental Orthodox Churches, the Assyrian Church of the East, and the Eastern Catholic Churches), these moveable feasts form what is called the Paschal cycle, which stands in contrast to the approach taken by Catholic and Protestant Christianity.
Most other feast days, such as those of particular saints, are fixed feasts, held on the same date every year. However, some observances are always held on the same day of the week, and thus occur on a range of days without depending on the date of Easter. For example, the start of Advent is the Sunday nearest November 30. In addition, the observance of some fixed feasts may move a few days in a particular year to not clash with that year's date for a more important moveable feast. There are rare examples of saints with genuinely moveable feast days, such as Saint Sarkis the Warrior in the calendar of the Armenian Church.
Moveable feasts in Christianity
Moveable feasts in Islam
Since Islamic feasts (Id ul Adha and Id ul Fitr) are lunar month based, they take place in different solar calendar dates and can occur at any time of the year.
- John Ayto Oxford Dictionary of English Idioms 2010 p123 019954378X "a movable feast an event which takes place at no regular time. In a religious context a movable feast is a feast day (especially Easter Day and the other Christian holy days whose dates are related to it) which does not occur on the same calendar date each year."
- A table of moveable feasts with dates, published by the Church of England.
- "Why Some Feasts Are Moveable", a Slate article
- "How the dates of moveable feasts are calculated, then and now", translated from the Latin by Michael Deckers.