The 13 Month Sol Calendar

Configuration:

Twelve contiguous and identical months of exactly 28 days.
The last, thirteenth month of the year (December) has 29 days normally and 30 days in a leap year.
The new month (Sol) occurs between June and July.

Features:

Avoids the use of intercalary days and the disruption of the standard week.
Implementations the 13 period financial calendar already in wide use in business and accounting.
Placing the new month (Sol) at mid-year minimizes the seasonal displacement of the traditional months.
Placing the extra day(s) year-end allows all months to be identical for their first 28 days in any given year.
Although not perpetual, the calendar does offer an easy method for date/day calculations.

Simple Calculations:

If you know the day of the week that January 1st occurred upon, it is quite simple to mentally calculate any day/date for the rest of the year. Subtract the highest possible multiple of 7 from the date in question and, beginning with the day of the week January 1st fell upon, count remainder (modulus) number of days for the answer.

Example: Assuming January 1st was a Friday, what day of the week does October 17th fall upon.

Answer: 17-(7*2)=3; beginning with Friday, count 3 days (Friday, Saturday, Sunday); October 17th is a Sunday.

You also know that the first of every month will fall on exactly the same day as in January, thereby making it trivial to calculate a date in any other month.

Example: Assuming January 1st was a Friday, what is the 3rd Monday in May?

Answer: The first Monday in January was the 4th, therefore the third Monday was January 18th. Since all the months are identical for the first 28 days, the third Monday in May would also be the 18th.

Examples:

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13 month sol calendar

The 13 Month Sol Calendar

Configuration:

Features:

Simple Calculations:

Examples: