Thirty days hath September, April, June, and November. And once, in Sweden, February. In 1712, Sweden had a day called February 30.

It all has to do with the shift from the Julian to the Gregorian calendars. The Julian calendar was designed by Julius Caesar in 46 BCE, and became the calendar of the Roman Empire, and hence the entire Western world. It is almost indistinguishable from our calendar, for most purposes. But there is a subtle, and very important difference. In the Julian calendar, every fourth year is a Leap Year. February normally has 28 days, but in Leap Years, it has 29.

Sounds like our calendar, right? Like I said, it almost is, but not quite. The whole idea of the Leap Year was to make up for the fact that the earth doesn’t go around the sun in exactly 365 days. It’s actually closer to 365.25. Without Leap Year, then slowly, very slowly, the seasons would start to drift off of where they belonged on the calendar. Every four years, they’d drift by one day, which means after about 120 years, the winter solstice would be falling around November 21, and summer would begin in May! After a few centuries, it would be hopelessly out of whack. So thanks to Caesar’s astronomers, and his own canniness, Caesar invented Leap Day. This would keep things running fine for quite some time. And they did.

But not forever. Because, you see, the earth doesn’t go around the sun in exactly 365.25 days. It’s actually closer to 365.2425. Astronomers knew that, even in Caesar’s day, but it didn’t become much of an issue for about sixteen centuries. By 1582, though, the Julian calendar had drifted 10 days off of the “true” equinoxes and solstices. And this did not make the Roman Catholic Church happy. This meant that the calculated date of Easter (which, you may recall, is always the first Sunday after the first full moon after the vernal equinox) wasn’t quite when it “should” have been. Pope Gregory XIII demanded a more accurate calendar, and the Gregorian calendar was created. It looks, on the surface, identical to the Julian calendar; but the computation of leap years is different. Here’s how leap years work in the Gregorian calendar (which is the one we still use today):

  • Every year that is a multiple of 4 is a Leap Year, unless
    • It’s also a multiple of 100, in which it’s not a Leap Year, unless
      • It’s also a multiple of 400, in which it is a Leap Year.

Or, more simply: every four years are Leap Years. But a “century year” (like 1800, 1900, etc.) is only a Leap Year if it’s a multiple of 400 (like the year 2000). This modest change meant that the new calendar was far more accurate, and wouldn’t lose any more days. So that’s half the problem solved – no more drift; the other half is how to make up for the drift that had already occurred. The easiest solution was to just change by papal bull: Thursday, October 4, 1582 was followed by Friday, October 15, 1582. Ten days fixed. Done and done.

Except it wasn’t. If only the church had done this a century earlier, before the Reformation, it would have been so much easier. But as it was, the pope had no authority over any countries that were Protestant, and they had no desire to just follow this crazy new calendar for Catholics. So it took a long, long, long time for all of Europe to follow suit. Great Britain didn’t do so until 1752, almost two hundred years later. Russia didn’t switch until 1918, and Turkey in 1926, 344 years after Spain, Italy, France and other Catholic countries. It must have been quite a mess to try to do business across national borders in those days. (Maybe Brexit will return the Julian calendar to Britain?)

Most countries just skipped ten days (or eleven, or twelve, depending on how late they did it, because the Julian drift kept getting worse) when they adopted it. But Sweden didn’t. In 1700, Sweden decided to switch to Gregorian, but they were committed to doing it in a dumb way. They decided, instead of skipping eleven days, they would skip the next eleven Leap Days, so that there would be no February 29 from 1700 through 1740. This would mean that they’d be out of step with everyone (Gregorian and Julian countries both) for four decades. Brilliant. And on top of that, they screwed it up, and forgot to skip the Leap Days in 1704 and 1708. Utterly brilliant. So King Charles XII decided in 1712 to fix it once and for all. How did he do it? He sent them back to the Julian calendar in 1712, by extending February to 30 days. Genius, just genius. In 1753, Sweden finally gave in, tail between its legs, and went from Wednesday, February 17 to Thursday, March 1.

Two interesting final notes: first, many Eastern Orthodox churches still use the Julian calendar to determine the dates of festivals. This is why “Orthodox Easter” is sometimes several weeks apart from “Western Easter.” It’s also why they celebrate Christmas on January 7. January 7 in the Gregorian calendar is December 25 in the Julian. (For now. After the year 2100, it will be January 8.)

And second, the earth doesn’t go around the sun in exactly 365.2425 days. It’s actually closer to 365.24217. We’re drifting away from the seasons at about one day every 3,000 years. So whoever is Pope in the year 31,000 or so is going to have to make some hard choices.

4 thoughts on “Thirty

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