This entire series revolves around it, so it’s only fair to dedicate some love to our modern calendar. When Pope Gregory XIII announced a reform to the old Julian Calendar 433 years ago on February 24, 1582, it looked like a minor tweak, compared to the major overhaul the Julian Calendar itself had been. The Julian Calendar had been in use since the Romans had introduced it over 16 centuries earlier, and for the most part, it was a good one. (The pre-Julian calendar it replaced in 45 BC, on the other hand, was a mess, with a wacky leap month of variable length inserted every few years.) The Julian Calendar was much more stable, with 365 days divided into 12 months, and an extra day squeezed in every four years. Sounds basically like today, right?

The problem was that the actual solar year is not a precise 365 ¼ days long, so the Julian Calendar overshot the Earth’s rotation around the Sun by about 11 minutes every year. This added up to an extra 3 days every 400 years. By Gregory’s time, the vernal equinox – which helped determine the date of Easter – had drifted about 10 days to March 11. Gregory knew that if he didn’t do something, Easter would eventually end up in December, and then Santa and the Easter Bunny would have some kind of holiday death match.

To restore Easter back to its rightful spot, Gregory issued a papal bull on this date reforming the calendar. Well, it was sort of on this date. If you’ve been paying attention, you know that the Julian Calendar, which dated Gregory’s pronouncement, had drifted by about 10 days, making this an anniversary only in a rough sense. This is a common problem with dating events before Gregory’s calendar reforms, and our options are basically to do a lot of extra math to match any Western date before 1582 to our modern calendar, or just live with it.

Gregory’s reform made three changes: One was to cut back on the leap years, which were pulling Easter further and further up in the year. Gregory eliminated 3 leap years every 400 years, which is still what we do today. (Years ending in “00” are only leap years if they can be divided by 400. The year 2000 was a leap year, but 1700, 1800, and 1900 weren’t.) The second change involved some complicated business with the lunar calendar which makes my head hurt to think about, but which was crucial to putting Easter back where the Nicene Council had placed it in 325. (On another note, calculating the date of Easter is really complicated.)

Finally, there was the issue of what to do with all those extra days that had built up under the Julian Calendar, like plaque in the calendar’s teeth. Gregory just waved them away, and people in the Catholic areas that adopted the calendar reforms right away went to bed on October 4, 1582 and woke up on October 15 (presumably very hungry and really, really needing to pee). It would take much longer for the Gregorian Calendar to catch on outside of Catholic Europe. Protestant and Orthodox countries held out for a while, but the logic of Gregory’s changes (and demands for convenience in international trade) eventually overcame religious suspicion. In 1923, Greece became the last European holdout to make the switch. Today the Gregorian Calendar is the standard for much of the world. But I’ll be honest with you, I still have no clue when Easter is. Sorry, Greg.

http://www.nta.ng/wp-content/uploads/2015/01/nta-image-gallery-calendar.jpg