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International Fixed Calendar
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From Wikipedia, the free encyclopedia
13-month calendar where every date is fixed to a day of the week
The International Fixed Calendar (also known as the Cotsworth plan , the Cotsworth calendar , the Eastman plan or the Yearal )[1] was a proposed reform of the Gregorian calendar designed by Moses B. Cotsworth, first presented in 1902.[2] The International Fixed Calendar divides the year into 13 months of 28 days each. A type of perennial calendar, every date is fixed to the same weekday every year. Though it was never officially adopted at the country level, the entrepreneur George Eastman instituted its use at the Eastman Kodak Company in 1928, where it was used until 1989.[3] While it is sometimes described as the 13-month calendar or the equal-month calendar, various alternative calendar designs share these features.
Rules<br>[edit]
The calendar year has 13 months with 28 days each, divided into exactly 4 weeks (13 × 28 = 364). An extra day added as a holiday at the end of the year (after December 28, i.e. equal to December 31 Gregorian), sometimes called "Year Day", does not belong to any week and brings the total to 365 days. Each year coincides with the corresponding Gregorian year, so January 1 in the Cotsworth calendar always falls on Gregorian January 1.[a] Twelve months are named and ordered the same as those of the Gregorian calendar, except that the extra month is inserted between June and July, and called Sol. Situated in mid-summer (from the point of view of its Northern Hemisphere authors) and including the mid-year solstice, the name of the new month was chosen in homage to the sun.[4]
Leap years in the International Fixed Calendar contain 366 days, and its occurrence follows the Gregorian rule. There is a leap year in every year whose number is divisible by 4, but not if the year number is divisible by 100, unless it is also divisible by 400. So although the year 2000 was a leap year, the years 1700, 1800, and 1900 were common years. The International Fixed Calendar inserts the extra day in leap years as June 29 – between Saturday June 28 and Sunday Sol 1.<br>Each month begins on a Sunday, and ends on a Saturday; consequently, every year begins on Sunday. Neither Year Day nor leap day are considered to be part of any week; they are preceded by a Saturday and are followed by a Sunday, making a long weekend.
As a result, a particular day usually has a different day of the week in the IFC than in all traditional calendars that contain a seven-day week. The IFC is, however, almost compatible with the World Calendar in this regard, because it also starts Sunday and has the extra day at the end of the year and the leap day in the middle, except IFC leaps on Gregorian June 17 and TWC leaps two weeks later on July 1. Since this break of the ancient week cycle has been a major concern raised against its adoption, various leap week calendars have been proposed as a solution.
Common arrangement of all months
Days of the week
Sun
Mon
Tue
Wed
Thu
Fri
Sat
Hol
01<br>02<br>03<br>04<br>05<br>06<br>07
Leap Day,<br>Year Day
08<br>09<br>10<br>11<br>12<br>13<br>14
15<br>16<br>17<br>18<br>19<br>20<br>21
22<br>23<br>24<br>25<br>26<br>27<br>28<br>X*
* The two special dates have been recorded as either the 29th day of the month ending or the 0th day of the month beginning, or, more correctly, as outside any month and week with no ordinal number.
The date for 27 June 2026, using this calendar is Tuesday, 10 Sol 2026.
The following table shows how the 13 months and extra days of the International Fixed Calendar occur in relation to the dates of the Gregorian calendar:
IFC
Matching dates on the Gregorian calendar
Starts on fixed day 1
Ends on fixed day 28
January
January 1<br>January 28
February
January 29<br>February 25
March
February 26<br>March 25*
April
March 26*<br>April 22*
May
April 23*<br>May 20*
June
May 21*<br>June 17*
Leap Day*
June 17
Sol
June 18<br>July 15
July
July 16<br>August 12
August
August 13<br>September 9
September
September 10<br>October 7
October
October 8<br>November 4
November
November 5<br>December 2
December
December 3<br>December 30
Year Day
December 31
* In a leap year, these Gregorian dates between March and June are a day earlier. March in the Fixed Calendar always has a fixed number of days (28), and includes a potential Gregorian February 29. The rule for finding leap years is the same in both calendars.
History<br>[edit]
Lunisolar calendars, with fixed weekdays, existed in many ancient cultures, with certain holidays always falling on the same dates of the month and days of the week.
The idea of a 13-month perennial calendar has been around since at least the...