In November 1699 it had been decided that Sweden would begin to adopt the New Style, or the Gregorian calendar, starting in 1700. The process would be to gradually reduce one day per year, over a period of 11 years. (Some sources say the plan was to skip all leap days in the period 1700 to 1740, thus gradually approaching the Gregorian Calendar over 40 years.) According to plan one leap day was reduced in 1700, but no further reductions were made in the following years. In January 1711, King Charles XII declared that Sweden would abandon the calendar, which wasn't in use by any other nation nor having achieved its objective, in favor of a return to the Old Style. An extra day was added to February in the leap year of 1712, thus giving it a unique 30 day length.
In 1753 Sweden introduced the New Style, whereby the leap of 11 days was accomplished by letting February 17 be immediately followed by March 1. Despite this, Sweden did not accept the Gregorian rules for determining Easter until 1844.
See also: Leap year