The International Fixed calendar (also known as the International Perpetual 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 calendar year has 13 months each with 28 days plus an extra day at the end of the year not belonging to any month. Each year coincides with the corresponding Gregorian year.

The months are named the same as for the Gregorian calendar except that a month called Sol or Midi is inserted between June and July.

In leap years, a leap day, also belonging to no month is inserted between June and the new month. Common 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 subsequent day that belongs to a month is 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).

Days that do not belong to a month are deemed to be outside the week and always occur between a day deemed to be Saturday and a day deemed to be Sunday.

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.

This causes all months to look like this:

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

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.

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