17 Feb 1753. Sweden’s transition to the Gregorian calendar was difficult and protracted. Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. Meanwhile, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every four years. This system had the potential for confusion when working out the dates of Swedish events in this 40-year period. To add to the confusion, the system was poorly administered, and the leap days that should have been excluded in 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should have been 8 days behind the Gregorian but was 10 days behind. King Charles XII recognized that the gradual change to the new system was not working, and he abandoned it.
Rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the solar portion of the Gregorian calendar in 1753, when Wednesday, 17 February, was followed by Thursday, 1 March. What became later Finland was an integral part of the Swedish kingdom at that time, hence it did the same.[15] The Russian Empire’s 1809 conquest of Finland did not revert this, since autonomy was granted, but government documents in Finland were dated in both the Julian and Gregorian styles. This practice ended when independence was gained in 1917.
Wikipedia