The year served to fix the time of festivals, which occurred at the end of each month. Very little is known about the calendar used by the Incas of Peru. Some scholars have even said they had no calendar. But most historians believe that the Incas had a calendar based on observations of both sun and moon. Names of 12 lunar months are recorded as well as their association with festivals in the agricultural cycle.
Work seems to have been organized on the basis of a nine-day week.
Three nine-day weeks, or 27 days, is the approximate time between new moons. Every third year was made up of 13 moons, with the others having 12. This formed a cycle of 37 moons, and 20 of these cycles made a period of 60 years. A true year, as opposed to a calendar year, may be defined as the time the Earth takes to return to the same point on its orbit around the sun. Astronomers therefore recognize different kinds of year.