Egyptian intercalary month

The intercalary month or epagomenal days of the Egyptian, Coptic, and Ethiopian calendars are a period of five days in common years and six days in leap years in addition to those calendars' 12 standard months, sometimes reckoned as their thirteenth month. They originated as a periodic measure to ensure that the heliacal rising of Sirius would occur in the 12th month of the Egyptian lunar calendar but became a regular feature of the civil calendar and its descendants. Egyptian and Ethiopian leap days occur in the year preceding Gregorian leap years.

The English names "intercalary month" and "epagomenal days" derive from Latin intercalārius ("proclaimed between") and Greek epagómenoi (ἐπαγόμενοι) or epagómenai (ἐπαγόμεναι, "brought in" or "added on"),Latinized as epagomenae. The period is also sometimes known as the "monthless days".

In ancient Egypt, the period was known as the "Five Days upon the Year" (Egyptian: Hrw 5 Ḥry Rnpt), the "Five Days" (Hrw 5) or "Those upon the Year" (Ḥryw Rnpt), the last of which is transliterated as Heriu Renpet.Parker also proposed that in some cases the intercalary month was known by the name Thoth (Ḏḥwtyt) after the festival that gave its name to the following month.

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