In the early fathers’ canons (Dideche), the Nativity of the Lord Jesus should be celebrated on the 29th of the fourth month of the Egyptian calendar, which is Kiyahk 29. Therefore, we celebrate it on that date every year until now. In the early centuries of Christianity, the 29th of Kiyahk used to coincide with December 25th, but after the reformation to the Julian Calendar in 1582 AD, the 29th of Kiyahk slipped by 10 days and continues to slip. Now it falls on January 7 of the Julian calendar.
Additionally, the church keeps the period between the Annunciation (29 Paramhat) and the Nativity (29 Kiyahk) to be exactly 275 days, which is the period of conception of Jesus in the womb of St Mary. However, on a Coptic leap year (every four years) the small month (Nasi) becomes six days rather than five. In this case, if we celebrate the Nativity on the 29th of Kiyahk, the period of conception becomes 276 instead of 275. Therefore, we celebrate the Nativity feast following a Coptic leap year on Kiyahk 28 to keep the conception period 275 days. The Nativity feast in this case is celebrated on the 28th and the 29th of Kiyah, but the readings remain that of the 29th of Kiyahk for both the 28th and the 29th of Kiyahk.
Notice that the celebration of the Nativity feast on the 28th and the 29th of Kiyahk (falling on January 7 and 8) occurs every four years and always falls on the year following the Coptic leap year.
From Virgin Mary Church in Zeitoun, Cairo. Egypt