Coptic, Ethiopic, Egyptian and Armenian Holidays Rules
There are 4 calendars based on the historical 'Egyptian' calendar system.
The Egyptian and Armenian calendars are of historical interest only (contrary to what their name implies, they are used neither in Egypt nor in Armenia).
The Coptic calendar is used by Copts in Egypt and upstream along the Nile valley, but contrary to the holiday names, such as Coptic Christmas, used in countries such as Egypt and Eritrea, the Coptic calendar is not used to calculate the date of these public holidays.
Only the Ethiopic calendar is currently used to determine the date of official non-working public holidays anywhere in the world. Holidays based on the Ethiopic calendar consist of a Month/Day pair.
See also: Recurrence Rules Options.