The Same Feast, Read Off an Older Calendar

Churches that kept the Julian calendar rather than adopting the Gregorian reform of 1582 continue to mark December 25 by their own liturgical calendar, a date that corresponds to January 7 on the Gregorian calendar most of the world now uses civilly, the date this rule records. The gap between the two calendars is not fixed, since the Julian calendar's leap year rule is slightly less accurate than the Gregorian one and the two drift further apart across the centuries, currently by thirteen days, which is why the same 25 December feast falls on 7 January by the newer calendar.

Churches keeping this reckoning today include the Russian Orthodox Church, the Serbian Orthodox Church, the Orthodox Patriarchate of Jerusalem, and a number of Eastern Catholic communities in communion with Rome that have nonetheless retained the older calendar for liturgical purposes, alongside several other Orthodox jurisdictions. It is worth naming plainly what this rule is not describing, a different theological date for the Nativity, since every church observing January 7 is keeping the identical December 25 feast the same Calendar of Filocalus tradition names, read through an older calendar rather than commemorating a different day in any doctrinal sense.