Koiak

Koiak (; , []), also known as Choiak (, Khoiák) and Kiyahk (, Kiahk, []; or ), is the fourth month of the ancient Egyptian and Coptic calendars. It lasts between 10 December and 8 January of the Gregorian calendar, or between 11 December and 9 January of the Gregorian calendar in Coptic calendar years immediately following a Coptic calendar leap year (which occur every four years, in Coptic calendar years immediately preceding those that are divisible by 4 to produce an integer; i.e., 1719, 1723, 1727, 1731, etc. are all examples of leap years in the Coptic calendar).

The month of Koiak is also the fourth month of the Season of Akhet (Inundation) in Ancient Egypt, when the Nile floods historically covered the land. They have not done so since the construction of the High Dam at Aswan.