What are the names of the two Aztec calendars? What do they keep track of? The first calendar, called xiuhpohualli, includes 365 days and measures the year by agricultural seasons. It includes 18 20-day months of 5-day weeks plus the five unlucky days. The second calendar, tonalpohualli, measures a sacred 260-day year of 13-day weeks. Each day has a name and is also assigned a number between 1 and 13.