Koiak /ˈkʰɔjæk/ (Coptic: Ⲕⲟⲓⲁⲕ Koiak /ˈkɔjak/ or Ⲕⲓⲁϩⲕ Kiahk /ˈkijaxk/), is the fourth month of the Coptic calendar. 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.
The name of the month of Koiak comes from *Kuʔ ḥar Kuʔ 'Soul upon Soul', a name of the Ancient Egyptian sacred Apis Bull.
The month of Koiak holds a special place in the rite of the Coptic Orthodox Church. It is known as the Mariam month (The month of Mary) because the Nativity according to the Coptic calendar falls on 29 Koiak. The month is characterized by beautiful midnight praises that commemorate the Lord's Incarnation and venerate His mother the Virgin Mary. The name of the Koiak midnight praise translates into Seven and Four, describing the outline of the praise that consists of 4 Canticles and 7 Theotokia (glorifications of Saint Mary).