The 13-Month Solstice Calendar

The year begins on the day of the winter solstice (defined here as December 21 in the Gregorian calendar) and consists of 13 months of exactly 4 weeks each. Year numbering is inherited unchanged from the international civil calendar, with the year shifted 11 days earlier.

The months are:

#	Month name	Code
1	Primarion	PMR
2	Secundarion	SCN
3	Tertarion	TRT
4	Quartarion	QTR
5	Quintarion	QNT
6	Sextarion	SXT
7	Septarion	SPT
8	Octarion	OCN
9	Nonarion	NNR
10	Decarion	DCR
11	Undecarion	UDC
12	Duodecarion	DDC
13	Tredecarion	TDC

Each month has exactly 28 days. Two special days do not belong to any month or week:

• Janday – the first (zero) day of the year, corresponding to December 21 in the Gregorian calendar. In numeric MM-DD form it is written as 01-00.
• Leapday – an extra day added at the end of Tredecarion in leap years (numeric 13-29). Like Janday, it has no weekday.

Because of this, every date always falls on the same weekday: for example, the 1st of any month is always Monday, the 7th is always Sunday, the 17th is always Wednesday, and so on.

This structure guarantees a perfectly regular calendar: no shifting weekdays, no irregular months, and a one-to-one correspondence between a date and its weekday.