13 Day Calendar. The international fixed calendar of 13 months containing 28 days each. Tochtli is the last trecena of the sacred year.

In the book he analyses european mythology and the irish ogham alphabet to propose that celts had a secret hidden lunar calendar of 13 months of 28 days, with an extra. Containing 13 identical months of 28 days each, this calendar includes an addtional short month at the beginning of each year containing a single day plus a leap year day every four (or so) years.