tillökningsdagen

Rather than share the turmoil experienced by other parts of Europe in terms of lost weeks when converting from the Julian to the Gregorian calendar, in 1699, the government opted to gradually transition to it over a planned four decades, skipping leap days and eventually sychronising the dates.
Due to complications of the Great Northern War with Peter I of Russia, however, this business of calendar reform couldn’t be promulgated properly across Sweden and only the twenty-ninth of February of 1700 was omitted. By 1712, realizing that the provisional calendar was more trouble than it was worth and the country was not only no closer towards alignment with the Gregorian calendar and was moreover a day behind the rest of the world still using the Julian date, Charles XII decreed that Sweden would add that missing day back and at least be synchronous with Orthodox and Protestant Europe, hence the unique 30th of February of that year.