Both based their calendars on the synodic period of Venus rather than a lunar calendar or solar calendar. This probably was inherited from an earlier group such as the Zapotecs. Not the Incas, who were over a thousand miles to the south in South America and had a very different calendar.
The correct answer should be religious rituals. They based the movements of the planets and the changes of seasons and days on ideas that it was gods doing it and so they had sacrifices for gods and festivals and things like that. The calendar was a religious thing, not a scientific one.
