Due to the inconvenient truth that the earth revolves ever-so-slightly more than 365 times during its annual orbit around the sun, every four years (called "leap years") we add an extra day to the calendar (called a "leap day") to keep it in sync with the cosmos. That day is Feb. 29.
This is famously aggravating to people (known as "leaplings") who were born on leap days, because they have to go through life celebrating their birthdays either a day early or a day late during non-leap years, when Feb. 29 doesn't exist.
Imagine, then, the frustration of being born on Feb. 30, a date that actually existed in Sweden in 1712. This is how that calendrical anomaly occurred.
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