Earth’s year isn’t technically 365 days, it’s about 365.25 days. Every four years we get an extra day tacked on to February.
Julius Caesar introduced this when he reformed the Roman calendar in the first century. But actually, I just lied to you. Earth’s year is actually 365.24219 days, which means that the Julian calendar is 11 minutes longer than the actual year. That might not seem like much, but over the centuries after Julius Caesar died, his calendar started drifting away from the solar year. In 1582, It was off by a full ten days!
So Pope Gregory XIII ordered that the calendar be fixed by emitting a few leap year every few centuries. Then he corrected the ten day error. And so October 4th, 1582, was followed immediately by October 15th, 1582. So instead of following the Julian calendar, today we follow the Gregorian calendar.But because the Pope is the one who made the change, the Orthodox Church didn’t get the update.
In the 20th century, Orthodox countries updated their civil calendars to match Gregory’s. But the Orthodox religious calendar is still the Julian calendar. Some of the Eastern Orthodox churches like Greece, Cyprus and Romania changed to the so-called revised Julian calendar in 1923. It does things like make Christmas match with the Gregorian calendar. But most other Orthodox churches, like in Serbia, Macedonia, Georgia or Belarus, still use the original Julian calendar. And since the 1500s, the Julian calendar has fallen back by another two days.
So if you missed the leap year celebrations today, you can look forward to the Julian Leap Day in 13 days.
