Nēmontēmi

In the Aztec culture, the Nahuatl word nēmontēmi 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 nēmontēmi accounted for the remaining 5 days of the approximate tropical year. According to the research by R. C. Tunnicliffe, the Aztecs dealt with the remaining fractional-day discrepancy with the true tropical year length by adding a trecena (13 days) after each bundle of 52 years; these 13 days were not considered unlucky, but they were not characterized by the features (numbers and symbols) of the Aztec calendar.

The word nēmontēmi means "days of reflection" 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 nēmontēmi. This is however, incorrect. People take time to reflect on the past year during these days and often includes a period of fasting.

References

R. C. Tunnicliffe, Aztec Astrology. This work shows how the nēmontēmi days are calculated for each current year (around 5 to 10 of April every year).