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The thirteenth month is inserted between June and July and called Sol or Midi. In leap years, a leap day, again belonging to no month or week is inserted between June and the new month. Regular years are 365 days long; leap years are 366 days long.
The International Fixed Calendar League, with offices in London and in Rochester, New York, ceased activities in the 1930s. In recent years there have been attempts to revive the plan.
The International Perpetual calendar is based on the Positivist Calendar published in 1849 by French philosopher Auguste Comte (1798-1857). Comte based his calendar on Polynesian calendars. The main difference between the International Perpetual calendar and the Positivist calendar is the names Comte gave to months and days. The months in the Positivist calendar were, in order: Moses, Homer, Aristotle, Archimedes, Caesar, St. Paul, Charlemagne, Dante, Gutenberg, Shakespeare, Descartes, Frederick II and Bichat. Every day of the year was likewise named. Positivist weeks begin with Monday instead of Sunday.
The 13 months and extra days occur on the following Gregorian dates:
January Jan 1 - Jan 28 July Jul 16 - Aug 12 February Jan 29 - Feb 25 August Aug 13 - Sep 9 March Feb 26 - Mar 25* September Sep 10 - Oct 7 April Mar 26* - Apr 22* October Oct 8 - Nov 4 May Apr 23* - May 20* November Nov 5 - Dec 2 June May 21* - Jun 17* December Dec 3 - Dec 30 Leap Day June 17 Extra day December 31 Sol/Midi Jun 18 - Jul 15 * These dates are a day earlier in a leap year