Why February 29 is a leap day

What happened?

Don't call it a bonus day. But every leap year, we get a 29th day in February.

How we got here

Julius Caesar, "dealing with major seasonal drift" in Roman calendars, introduced the Julian calendar in 46 BCE, counting a year as 365.25 days and adding an extra day every four years, The Associated Press said. But a year is actually 365.242 days, so Pope Gregory XIII knocked out a leap year every hundred years — except years divisible by 400, like 2000. The 1582 Gregorian calendar "remains in use today" and, while its "gnarly math" is not perfect, it reduced "drift to mere seconds."

The commentary

"Without the leap years, after a few hundred years we will have summer in November" and Christmas in summer, said Younas Khan, a physics instructor at the University of Alabama at Birmingham. Some people treat Feb. 29 "as a free day" to do things "they've long been putting off," Phil Plait said at Scientific American. "I think that's a pretty good idea because, after all, catching up is what leap day is all about," astronomically speaking.

Subscribe to The Week

Escape your echo chamber. Get the facts behind the news, plus analysis from multiple perspectives.

SUBSCRIBE & SAVE

Sign up for The Week's Free Newsletters

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

From our morning news briefing to a weekly Good News Newsletter, get the best of The Week delivered directly to your inbox.

Sign up

What next?

There will be leap days every four years until the next skip year, 2100 — or till the Gregorian calendar is changed.