The calendar Gregorian replaced — still setting the date of Orthodox Christmas.
The Julian calendar, introduced under Julius Caesar in 45 BC, adds a leap day every four years without exception. That slightly overstates the solar year, and by the sixteenth century the accumulated error had moved the equinox ten days.
The Gregorian reform corrected it, and states adopted the correction across three centuries — Britain in 1752, Russia in 1918, Greece in 1923. Several Orthodox churches never did, and continue to reckon feasts by the Julian calendar.
The practical residue is a thirteen-day offset. Christmas on 25 December Julian falls on 7 January Gregorian, which is why several countries hold a public holiday in January for a feast most of Europe kept a fortnight earlier.
The same feast, two calendars. Each of these is 25 December in the Julian reckoning, converted to the Gregorian date a system would actually use.
| 🇧🇦 | Thursday 7 January 2027Bosnia and Herzegovina | Orthodox Christmas25 December by the Julian calendar, thirteen days behind the Gregorian. |
| 🇧🇾 | Thursday 7 January 2027Belarus | Orthodox ChristmasThe same offset, a public holiday. |
Gregorian is the civil calendar; Julian is what it replaced. The thirteen-day gap between them is why Orthodox feasts land in January.
See also gregorian calendar · ethiopian calendar · leap year
We hold this as rule_type: julian_date and relative_to_orthodox_easter. See what we hold.