The Annuary Calendar is a Calendar invented by Karl Palmen in which each year has twelve regular months alternating between 29 and 30 days plus a 30-day intercalary month occurring once every 33 months. Such a calendar has an 8-year cycle of 99 months with a mean year of 365.25 days. To reduce the mean year to 365.2425 days three intercalary months every 400 years have 29 days (instead of 30 days). The mean month is a little short of a lunar month and the calendar has a rule to reckon the slow drift of the moon phases against the calendar month.