Why February Only Has 28 Days (Usually)

Today is a rare day — February 29th. While the other eleven months have a 29th and 30th day every year, February only gets one once every four years, and (almost) never has a 30th day. Most often, February 28th is the last day of the year.

That seems dumb. Why does February only have 28 days in most years?

It’s a combination of seasonal change, the moon, and a bit of superstition for good measure.

The calendar we use today is based on early Roman calendars. The first calendar, per Roman lore, was established by Romulus, the first King of Rome, who reigned from 753 to 716 BCE. The history of that calendar, like the details of Romulus’ life, is masked by legend, and we can’t be sure where the line between fact and fiction sits. But it’s widely agreed that the first Roman calendar was only ten months long. The ten months corresponded to the current months of March through December; there was neither a January nor a February. (For purposes of today’s newsletter, I’m going to use the modern English names for the months, not previous names. It’s just easier that way.) Each month was either 30 or 31 days, a length chosen to match the moon cycle, which is roughly 29.5 days. (It’s not entirely clear where that extra day or so came from; some historians believe that the last day of one month and the first of the next were considered the same day.)

If you’re doing the math in your head, you’ll realize it doesn’t add up. That first Roman calendar only had 304 days, which isn’t nearly enough time for the Earth to make its annual transit around the sun. But the Romans didn’t care. The calendar was designed to help people plan their harvests — it began around the spring equinox in March and carried forward until it became too cold to reliably grow anything. When the end of the year hit, the calendar stopped mattering. As Readers Digest explains, “To the Romans, who made their lot by planting and harvesting, winter was a nameless, dateless slog. For part of the year, there was literally no system for keeping track of the days.” There was a 60-day gap of monthlessness.

When Numa Pompilius succeeded Romulus as the Roman king, he decided to do something about that gap. That meant inventing two new months, as Slate summarizes: Numa “decided to make the calendar more accurate by syncing it up with the actual lunar year—which is about 354 days long. Numa tacked on two months—January and February—after December to account for the new days.” To do some more math, that meant adding about 50 days, which, if you divide it by two, comes out to 25 days per month. That doesn’t quite work out to that 29.5-day moon cycle, so Numa reduced the 30-day months to 29 days, and then added January and February at 28 days each. That came to 354 days, which is exactly the length the lunar calendar should have been.

But Numa wasn’t done. At the time, Roman superstition held that odd numbers were lucky but even numbers? Not so much. Having a calendar with an even number of days would have been unlucky, so Numa decided to go with a 355-day calendar. Unfortunately, that created an unavoidable math problem: if you have an even number of months, an odd number of them has to have an odd number of days, otherwise, you’ll end up with an even number of days in the year. So, you need at least one month with an even number of days. And as the Straight Dope notes, “February got elected. It was the last month of the year (January didn’t become the first month until centuries later), it was in the middle of winter, and presumably, if there had to be an unlucky month, better to make it a short one.”

Numa Pompilius’ calendar still had a big problem — because the Earth takes 365 days (and change) to orbit the sun, his 355-day calendar fell out of sync with the seasons. According to Mental Floss, “To keep things straight, the Romans would occasionally insert a 27-day leap month called Mercedonius. The Romans would erase the last couple days of February and start the leap month on February 24—further evidence no one ever cared much for the month.” And when Julius Caesar redid the calendar a few centuries later, he decided to further enshrine February as the least of all the months, keeping it at 28 days — except for every four years, when he gave February that 29th day. And we’ve been stuck with it ever since.

Bonus fact: In 2008, the European Organization for Rare Diseases (EURORDIS) began a campaign to raise awareness of diseases that are very uncommon and, typically, do not have a lot of treatments available. (According to a 2016 report, “of the 7,000 diseases classified as ‘rare,’ only about 400 have an effective treatment,” in large part due to the lack of demand for treatments and the resultant lack of funding for research.) Their idea? A day dedicated to awareness of these conditions — and not just any day. EURORDIS designated the last day of February as Rare Disease Day, which in most years, falls on February 28th. But every four years, it falls on February 29th, which is not-so-coincidentally the rarest date there is.

From the Archives: When the Day After Friday is Friday: Another plan to tinker with the calendar.