30th of February 1712

At "Shut up and write" today I started research the clothing of the Reader of Tynwald. There' wasn't much available. Instead, I read about the date of Tynwald.

The Isle of Man switched from the Julian to the Gregorian calendar in 1753, a year after the United Kingdom and the Empire. At this time the calendars were 11 day out of synch (we are now at 13). Tynwald Day was a midsummer celebration on St John's day, the 24th of June. It was decreed Tynwald Day would be celebrated on the corresponding day in the Gregorian calendar, and that's why Tynwald Day is now celebrated on the 5th of July.

This then led me down a rabbit hold of Julian/Gregorian dates, and ripples in space and time, well, time. The one lodged in my brain is the 30th of February. It is a Swedish date, and only one year has had a 30th.

Countries around the world adopted the Gregorian system in a piecemeal fashion. Pope Gregory issued a papal bull decreeing the change in 1582. This was to correct the drifting of the calendar. The Julian calendar was out by around one day every century, plus a little bit. The Gregorian calendar is correct to one day every 30,030 years! The change was adopted immediately by the Catholic European countries and their colonies, although the colonies brought it in retroactively due to the time delay in communication. Protestant European and Orthodox countries made the change individually.  

Every country achieved this by skipping dates. In the first wave, Thursday, the 4th of October 1582 as followed by Friday, the 15th of October. Sweden took a different route, and decided to simply eliminate all the leap years from 1700 to 1740. These 11 'lost' days would bring them in line with the Gregorian calendar, but would mean they would be out of step with everyone for those 40 years.

It started well. but the Great Northern War interrupted the transition, and 1704 and 1708 had leap years. In 1711, Karl XII declared Sweden would return to the Julian system. Having not counted the 29th of February in 1700, they were a day out, and decided to tack it on after the 1712 leap day. This is how the 30th of February came to be.  

Sweden finally adopted the Gregorian dates in 1753, when Wednesday, 17 February, was followed by Thursday, 1 March.