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The 13-month Sol Calendar is a proposal for calendar reform by Jim Eikner of Austin, Texas, USA, providing for a year of 13 months in length, with 12 contiguous months having 28 days each, with the final month of the year, December, having 29. December would have 30 days in leap years.

The months are named the same as they are in the Gregorian calendar, except that a month called Sol is inserted between June and July.

Although every month of the same year begins on the same day of week, the months usually begin on different days of the week in different years. The calendar for every month in the year 2007 (and every other year that begins on a Monday) is as follows:

Week 1 2 3 4 5 6 7
Sun Mon Tue Wed Thu Fri Sat
1 01 02 03 04 05 06
2 07 08 09 10 11 12 13
3 14 15 16 17 18 19 20
4 21 22 23 24 25 26 27
5 28 29*
Week 1 2 3 4 5 6 7
Sun Mon Tue Wed Thu Fri Sat
1 01 02 03 04 05 06 07
2 08 09 10 11 12 13 14
3 15 16 17 18 19 20 21
4 22 23 24 25 26 27 28
5 29 29* 30†
  • * December 29.
  • In leap years December 30 is added. The next leap year beginning on Sunday would be 2040.

In 2018 all months would begin on Monday, in 2019 all months would begin on Tuesday, in 2020 all months would begin on Wednesday and in 2021 all months would begin on Friday..

The 13 months occur on the following Gregorian dates:

January Jan 1 - Jan 28
February Jan 29 - Feb 25
March Feb 26 - Mar 25*
April Mar 26* - Apr 22*
May Apr 23* - May 20*
June May 21* - Jun 17*
Sol Jun 18* - Jul 15*
July Jul 16* - Aug 12*
August Aug 13* - Sep 9*
September Sep 10* - Oct 7*
October Oct 8* - Nov 4*
November Nov 5* - Dec 2*
December Dec 3* - Dec 31

* These dates are a day earlier in a leap year

The 13-month Sol Calendar is an adaptation of International Fixed Calendar, which has its roots in the Positivist Calendar created by French philosopher August Comte in 1849.

Comte named months for famous people and also named each day, but Eikner’s calendar does not follow that example.

Eikner’s proposal also differs from these other 13-month calendars in that it is not perpetual. In each year, the 29th and 30th days of December are normal days of the week and so move the first day of the January of the following year either one or two days later in the week, in relation to that previous year. However, in any given year, monthly calendars for January through November are identical and every month begins on the same day of week.

The Positivist and International Fixed Calendars suggest adding "intercalary" days between months or at the end of the year, in order to have each month begin on the same day of week - year after year. Because these "off-calendar" days are not counted as part of any month or week, they are said to disrupt the ordering of the days of the traditional seven-day week. This has led these calendar proposals to face opposition by religious groups who are required to worship on a certain day of week, and also worship every seven days without exception.

Features and benefits[]

  • This calendar is named after its new month, Sol
  • Twelve contiguous and identical months of exactly 28 days
  • The thirteenth month of the year (December) has 29 days normally and 30 days in a leap year
  • This calendar has the same names for the months as the International Fixed Calendar, including the new month, Sol
  • The new month (Sol) occurs between June and July
  • Preserves the standard, 7-day week
  • Preserves Gregorian leap year rule
  • Almost all professional accounting systems offer a 13-period reporting option. Computer accounting programs will likely be easily adjusted to 13 months
  • Placing the new month (Sol) at mid-year minimizes the seasonal displacement of the traditional months
  • Placing the extra day(s) at the end of the year allows all months to be identical for their first 28 days in any given year
  • Leap years are every four years (in the same years we currently have leap years) and they add a 366th day to December (a Dec. 30), the same number of days the Gregorian calendar adds during a leap year
  • 2006 would begin on a Sunday, 2007 on a Monday
  • It preserves all of the current month’s traditional names

External links[]