Coptic Christmas is observed on what the Julian Calendar labels 25 December, a date that currently corresponds with 7 January on the more widely used Gregorian Calendar (which is also when Christmas is observed in Eastern Orthodox countries such as Russia).
The 25 December Nativity of Christ was attested very early by Hippolytus of Rome (170–236) in his Commentary on Daniel 4:23: “The first coming of our Lord, that in the flesh, in which he was born at Bethlehem, took place eight days before the calends of January, a Wednesday, in the forty-second year of the reign of Augustus, 5500 years from Adam.”
There may have been more practical considerations for choosing 25 December. The choice would help substitute a major Christian holiday for the popular Pagan celebrations surrounding the Winter Solstice (Roman Sol Sticia, the three-day stasis when the sun would rise consecutively in its southernmost point before heading north, 21, 22 and 23 December.
Until the 16th century, 25 December coincided with 29 Koiak of the Coptic calendar. However, upon the introduction of the Gregorian calendar in 1582, 25 December shifted 10 days earlier in comparison with the Julian and Coptic calendars.
Furthermore, the Gregorian calendar drops 3 leap days every 400 years to closely approximate the length of a solar year. As a result, the Coptic Christmas advances a day each time the Gregorian calendar drops a leap day (years AD 1700, 1800, and 1900).
This is the reason why Old-Calendrists (using the Julian and Coptic calendars) presently celebrate Christmas on 7 January, 13 days after the New-Calendrists (using the Gregorian calendar), who celebrate Christmas on 25 December. From AD 2100, the Coptic Christmas will be on the Gregorian date of 8 January.
| No. | Julian Calendar Dates | Gregorian Calendar Dates (1900-2099) | |
| 1 | August 29 – September 27 | 11 September – 10 October | |
| 2 | September 28 – October 27 | 11 October – 9 November | |
| 3 | October 28 – November 26 | 10 November – 9 December | |
| 4 | November 27 – December 26 | 10 December – 8 January | |
| 5 | December 27 – January 25 | 9 January – 7 February | |
| 6 | January 26 – February 24 | 8 February – 9 March | |
| 7 | February 25 – March 26 | 10 March – 8 April | |
| 8 | March 27 – April 25 | 9 April – 8 May | |
| 9 | April 26 – May 25 | 9 May – 7 June | |
| 10 | May 26 – June 24 | 8 June – 7 July | |
| 11 | June 25 – July 24 | 8 July – 6 August | |
| 12 | July 25 – August 23 | 7 August – 5 September | |
| 13 | August 24 – August 28 | 6–10 September |