The 13-month calendar was devised by Auguste Comte in 1849. It was based on a 364-day year which included the one or two "blank" days that Abbé Mastrofini, an Italian Roman Catholic priest, had devised 15 years before. Each of the 13 months had 28 days and exactly four weeks.

This "Positivist" calendar, however, was so belabored with names of prominent men and women from ancient to modern times that it received a cold response. About the beginning of the twentieth century, the plan was revived by Moses B. Cotsworth who eliminated the superfluous names, thereby simplifying the plan. The new thirteenth month was designated "Sol."

This perpetual calendar, better known in the United States as the Eastman plan, met with favorable reception among many Americans. It was the most rational plan which they knew (before The World Calendar Association launched The World Calendar Plan at the close of 1930) and thus was endorsed by quite a number of prominent business men, statisticians and accountants. The 13-month calendar was endorsed because of its seeming simplicity and the much-desired stability. The public, however, did not take kindly to it because the changes were too drastic. A few of the changes follow:

The 13-month calendar was not easily divisible. The first quarter-year would always end with April 7 and the second quarter would begin with April 8. The half-year would begin with the 15th of the new month, Sol, and the last quarter would begin on September 22. This arrangement gave to each quarter three months and one week, or three and one-quarter months, causing fractional months. This fact alone would cause inconvenience to all activities and businesses based on quarter-years.

Another feature, looked upon with disfavor in the United States, was the change of familiar dates such as the Fourth of July, which became Sol 17. Then, too, the complete upheaval that would result from the adoption of this calendar was overwhelming in its effect. For instance, only the first 28 days of January would remain and thus be comparable with the present calendar. . . . The League of Nations eliminated [the 13-month calendar] when in 1937 the Council submitted only The World Calendar to the various nations for their opinion.

After having fought vainly for many years for the adoption of the 13-month calendar, under the able leadership of George Eastman in the United States and Moses B. Cotsworth in England, the International Fixed Calendar League, with offices in London, England, and in Rochester, New York, quietly closed its doors about 1937, and activities ceased.