Not all 13-Moon calendars are created equal:
In terms of calendar reform, this is the pony to bet on.
13 months of 28 days.
(good introduction to 13 month calendars here.)
An intercalary day out of time is observed on July 25th, coincident with the heliacal rising of Sirius.
here’s a fascinating essay to account for the fraction of a day that completes every year (.241896698 of a day), and a means of using Mercury & Saturn’s orbit to calculate the leap day.
13 months of 28 days.
An extra month of 7 days is inserted after the new month, Columbus, between November and December as an alternative to calculating
146, 097 days in 400 years.
The extra month is inserted provided:
* the last one or two digits of the year are divisible by 6
* the year ends in 99
* the year is divisible by 100 but not by 400
13 months of 28 days.
the intercalary day takes place on the Winter Solstice.
has an Accumulator Number (AN). After each month the AN increases by 43. If the AN exceeds 450 in any given month, an extra day is added.
the math behind this calculation remains hidden.
13 months of 28 days
A leap week is inserted provided:
* the year is divisible by 5 or 400, but not by 40.
12 months of 28 days, 1 month of 29 or 30 days.
the year begins on the Winter Solstice, denoted as January 1st, which is always a Monday.
December 29th and 30th are an extended weekend without day names. Although it functions similarly to an intercalary day, technically, these are observed on the calendar.
13 months of 28 days.
the intercalary day takes place on July 20 (Armstrong Day).
