What is "nemontemi"

In the Mesoamerica culture, the nemontemi was the five days between two 360-day years, or between 52 year calendar rounds (xiuhmolpilli in Nahuatl, " New Fire ceremony"). It was also called baldíos (wasted).

As appears in the Codex Tovar:

Every month had 20 days and at the end of the cycle, adding up 360 days. The five days remaining adjusted the next following calendar round.