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 of 13 period financial calendars are 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.

Peculiarities: 

* Any year beginning on a Sunday will feature thirteen Friday the 13ths. 
* Blue moons (a second full moon in a calendar month) will become even 
  more rare since they can only occur in December. 
* More birthdays will be celebrated on the 28th over the next century, but 
  in declining numbers. 

Although not perpetual, the calendar does offer an easy method for 
date/day calculations:

To calculate any day or date in a given year you only need three bits of 
information. First, what day of the week did January 1st fall on? 
Secondly, is this a leap year? Lastly, remember the numbers 1, 8, 15 and 
22 (1, 1+7, 1+14, 1+21).

Assume January 1st was a Friday and it's a leap year. That means that 
the first, eighth, fifteenth and twenty-second day of every month is a 
Friday. The twenty-ninth day of December (1+28) is also a Friday and, 
since it's a leap year, Saturday the 30th is the last day of the year.

What is the third Monday in May? Again assuming January 1st was a 
Friday, the third Friday in any month is the 15th so, with a bit of 
addition, the third Monday is the 18th.

What day of the week is August 13th? August 8th is a Friday (as is 
August 15th) so either adding or subtracting, one deduces the date is a 
Wednesday.

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