The International Perpetual calendar (also known as the International Fixed calendar, the Cotsworth plan, the Eastman plan, the 13 Month calendar or the Equal Month calendar) is a proposal for calendar reform providing for a year of 13 months of 28 days each, with one day at the end of each year belonging to no month or week.

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 first day of each year, January 1, is deemed a Sunday and every day after that through the last day in June (in the International Perpetual calendar) is deemed to be the day of the week that comes after the day of the week which the previous day was deemed to be in the conventional 7-day week (the days of which go, in order, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday, followed by Sunday of the next week). The first day in Sol/Midi, which in every year but a leap year is the day after the last day in June (which is deemed a Saturday), is deemed a Sunday, and every day after that through the last day in December is deemed to be the day of the week that comes after the day of the week which the previous day was deemed to be in the conventional 7-day week. Since each month consists of exactly four weeks, the first day of each month and every seventh day after that for the rest of the month is deemed to be a Sunday, the second day of each month and every seventh day after that for the rest of the month is deemed to be a Monday, and so on. So each month begins on a Sunday and ends on a Saturday, just like each conventional week.

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, and Postivist months, 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

External link

• Article on the International Fixed Calendar League at the International Institute of Social History Archives.