Every fourth year on the Gregorian calendar is a "leap year" — one in which an extra day (called a "leap day") is added to make a year 366 days long instead of 365. Why? Because the earth actually rotates on its axis slightly more than the 365 times a year allowed for on the calendar during its annual orbit around the sun. Observing a leap day every four years is the "workaround," if you will, meant to keep the calendar in sync. That day is Feb. 29.
Therefore, there exist people, known as "leaplings," unlucky enough to have been born on a leap day, meaning they go through life having to celebrate their birthday either a day earlier or a day later — or not at all — during non-leap years, because leap days don't exist in between leap years.
Imagine, then, the frustration of being born in Sweden on Feb. 30, 1712, a date that actually occurred, but only once in history. This is the story of how that anomaly came about.
In 1582, Pope Gregory XIII introduced the Gregorian calendar to correct discrepancies in the existing Julian calendar, which had been in use since the time of Julius Caesar. Historically, calendar reform — which is accomplished, basically, by adding or subtracting days, weeks or even months — was never easy or readily accepted by a populace long accustomed to regulating its religious, official and commercial affairs under another system. It took several hundred years for the Gregorian calendar to be universally adopted — kingdom by kingdom, nation by nation — and there were misadventures along the way.
Take Sweden. The Swedes didn't jump on the Gregorian bandwagon until 1700, and even then proceeded slowly and in a piecemeal, even haphazard, fashion. As Roscoe Lamont explained in Popular Astronomy:
The year 1700 was a leap year by the Julian calendar but not by the Gregorian, and therefore March 1, 1700, Julian, corresponded to March 12, 1700, Gregorian, the difference then amounting to eleven days. Sweden, however, decided to stay just ten days behind, the same as she had been for over a hundred years, and therefore made the year 1700 a common year. The years 1704 and 1708 were leap years, and in 1712, also a leap year, another day was added to February to compensate for the one omitted in 1700, thus giving 367 days to the year 1712, February having thirty days. The extra day was added in February so that Easter would be celebrated on the Sunday designated by the Julian calendar-rules, it having been found that in 1705, 1709 and 1711, Easter day as observed in Sweden fell one week before the day observed in England where the Julian calendar was [still] used, and this would have happened again in 1712 if another day had not been added.
What it boiled down to was that Sweden resynchronized itself with the old Julian calendar by adding an extra leap day, Feb. 30, to 1712. Easter Sunday was restored to its proper day. But imagine you were a Swede born on Feb. 30, 1712. When would you have celebrated your birthday? Or imagine you were Sven Hall and Ellna Jeppsdotter, who, according to The Wall Street Journal, got married in Ystad, Sweden, on Feb. 30, 1712, and were never able to celebrate their wedding anniversary on the actual date for the rest of their lives.
When the country finally did adopt the Gregorian calendar in full in 1753, 11 days had to be excised from the month of February, with the result that the calendar skipped directly from Feb. 17 to March 1 that year. Eleven days simply vanished into thin air.
More: Julius Caesar: He Came, He Saw, He Conquered the Calendar and Gave Us New Year's Day