As scientific advances gave a more accurate measurement of the rotation and movement of the Earth, the Western Christian world found that the Julian calendar it had been using was too inaccurate. Compared to observed seasons and equinoxes, the Julian calendar would gain approximately three days every four centuries. After twelve centuries since the timing of Easter was set by the Council of Nicaea, the Council of Trent In 1563 set out a plan to correct and fix when Easter fell by modifying the calendar. Over the next twenty years, mathematicians worked out how to correct the issues and presented a new calendar. It kept the number of days per month and the quadrennial leap year from the Julian calendar. However, the new Gregorian calendar removed leap years from centenary years that were not multiples of 400. It also corrected the 11-day discrepancy that had built up since the Council of Nicaea. Pope Gregory XIII after whom the Gregorian calendar was named issued a papal bull for the calendar’s adoption on February 24, 1582. The Gregorian calendar is now the standard calendar used by much of the world today.
However, since the law establishing the Gregorian calendar was a papal bull, it initially only had effect in the Papal States and in use by the Catholic Church. The changes to the European civil calendars only came over the following centuries. The Catholic realms in Spain, Portugal, the Polish-Lithuanian Commonwealth, and the minor states in Italy adopted the new calendar on the date it was implemented in the Church. So in those countries, the date skipped from Thursday October 4, 1582 straight to Friday October 15. France, the Netherlands, and the Spanish and Portuguese possessions in the Low Countries and the New World adopted the calendar at various times over the following year. Protestant countries resisted the new calendar, and almost all of them did not adopt the Gregorian calendar until the 1700s. Great Britain and her colonies were the last country to adopt the calendar in 1753.
While most countries made the switch over the Gregorian calendar immediately, skipping all 11 days in one large chunk, Sweden decided to implement the change gradually by removing all leap days between 1700 and 1740. The first year went well for Sweden, with February 29, 1700 omitted from the calendar. However, the outbreak of the Great Northern War put more important matters at hand and no days were removed in later years. This put Sweden on its own calendar completely separate from the Julian and Gregorian calendars until 1712 when the scheme was abandoned. Sweden’s abandonment of the switch over and their return to the Julian calendar also created an interesting quirk. To add the lost day back and return to the Julian calendar, 1712 had a day added to it. Since it was already a leap year, another day was simply added to February. This makes 1712 the only year to have a February 30, and only in Sweden. Sweden finally flipped over to the Gregorian calendar in 1753.