The Nativity Fast (Advent or Winter Lent) is the 40 days preceding the Nativity of Christ (Christmas) on 29 Koiak (January 7, which also falls on 28 Koiak in leap years). The 40 days correspond to the 40 days that the Prophet Moses fasted on the mountain, before receiving the Ten Commandmentsfrom God, which were at that time considered the word of God to his people. Thus, with Christ being the Word of God, the Christians fast those 40 days in preparation of receiving the Word of God in flesh at the Feast of the Nativity.

An additional 3 days were added at the beginning of the 40 days of Advent during the 10th century AD to commemorate the 3 days that the Copts fasted before God awarded them the miracle of moving the Mokattam Mountain, which lies within a suburb of Cairo on the hands of Saint Simon the Tanner during the ruling of the Muslim Fatimid Caliph Al-Muizz Li-Deenillah. Thus, the fast of Advent begins on 16 Hathor (November 25, which also falls on 15 Hathor in leap years). The three added days are considered a separate fast rather than part of the fast of Advent.