Calendar 1 - The Xiuhpohualli. The first calendar of the Aztec people was called the xiuhpohualli, the counting of years. This was a 365 day year, of course very helpful for planning your farming and predicting the weather. There were 18 months, each 20 days long, or 4 (5 day) weeks.he 13 day period is a kind of Aztec week. Not only was every day ruled by a god, each of the weeks were also ruled by a god-the one associated with the first day. Find out today's date, or your birthday according to the Aztec calendar stone on this site ! More on the background and origins of the Aztec calendar.