Ethiopian calendar (thirteenth month)

So I wanted to learn why I keep reading thirteenth month when researching Ethiopia. Very interesting so I thought I would share :-)

Ethiopic calendar is the principal calendar used in Ethiopia and is also the liturgical year of the Eritrean Orthodox Tewahdo Church where it is known as the Ge'ez calendar. It is based on the older Alexandrian or Coptic calendar, which is based on the even older Egyptian calendar, but like the Julian calendar, it adds a leap day every four years without exception, and begins the year on August 29 or August 30 in the Julian calendar.

Like the Coptic calendar, the Ethiopian calendar has twelve months of 30 days each plus five or six epagomenal days (usually called a thirteenth month). Furthermore, its months begin on the same days as those of the Coptic calendar, but they have different names, that are in Ge'ez. The sixth epagomenal day is added every four years without exception on August 29 in the Julian calendar, six months before the Julian leap day. Thus the first day of the Ethiopian year, 1 Mäskäräm, for years between 1901 and 2099 (inclusive), is usually September 11 (Gregorian), but falls on September 12 (Gregorian), in years before the Gregorian leap year.