In the Aztec culture, the Nahuatl word nemontemi refers to a period of five intercalary days inserted between years of the Aztec calendar. Each of the 18 Aztec "months" had 20 days, for a total of 360 days. The nemontemi accounted for the remaining 5 days of the approximate tropical year. It is not known how (or even if) the Aztecs dealt with the remaining fractional-day discrepancy with the true tropical year length.
The word nemontemi means "useless days" or "unlucky days"; Spanish lexicographers glossed it as "dias baldios", "wasted days". They were considered to bring ill fortune, and most activities (including even cooking) were avoided if possible during the nemontemi.