The answer to this question is simpler than you may think—some Eastern Orthodox churches use a different calendar, called the Julian Calendar, for their religious holidays and feasts, to the Gregorian Calendar.
According to Wikipedia, “The Julian calendar was proposed in 46 BC by (and takes its name from) Julius Caesar as a reform of the earlier Roman calendar, which was largely a lunisolar one. It took effect on 1 January 45 BC, by his edict. Caesar’s calendar became the predominant calendar in the Roman Empire and subsequently most of the Western world for more than 1,600 years, until 1582 when Pope Gregory XIII promulgated a revised calendar.”
The Julian calendar year (365.25 days) is longer than the Gregorian by a few hours, as the solar year is approximately 365.2422 days. This means that the Julian calendar gains about 1 day in every 129 years. When Pope Gregory proposed the calendar change, he added 10 days to the new calendar in order to realign with the vernal equinox, and the change was put into effect by advancing the calendar by 10 days. Therefore, after October 4 in 1582, the day following was reckoned as October 15, 1582.
Let’s do the math. From 1582 until the present, 2025, the 443 years have resulted in 3 extra days in the Julian calendar. Adding that to the 10 days added by Pope Gregory, the difference in the number of days should be 13.
Therefore, while Christians have agreed that Christmas is celebrated on the 25th of December, according to the Julian calendar, the 25th of December falls on the 7th of January on the Gregorian calendar.
For more articles based on the Orthodox Churches’ traditions, follow this link:
https://copticapologetics.com/category/tradition/
