Vague Calendar of the Maya
The Maya's solar calendar is called "vague" because it only approximates the 365 day calendar. Each of its 18 months has 20 days. The final or twentieth day of each month makes use of the Maya's concept of zero: Instead of its being numbered "twenty," it is described as the day of the seating of the following month. That is, it is an empty or zero spot that is waiting for the new month.
The months of this agricultural calendar are: