In a menologic episode of Dan Lewis‘ new video series Now I Know, host Matt Silverman explains why in 1712, Sweden added an additional day at the end of February. The reason for doing so had to do with the country’s effort to complete their transformation from the Julian calendar to the Gregorian calendar, albeit the transition was a bit more complicated than that.

They decided to ignore all the leap years between 1700 and 1740 thereby losing 11 days over four decades on March 1st 1740 the Swedes and the rest of the West would synch perfectly. Except they screwed it up for some reason they forgot to skip the leap years in 1704 and 1708 …The Swedish king decided to fix the problem in 1712, but instead of just dropping 10 days and joining the Gregorian calendar. in one shot he went back to the Julian calendar and added two leap days to 1712 to make up for the missing day in 1701 was the regular Julian leave day and the other, as seen in a Swedish Almanac, from that year is February 30th.