This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Nemontemi is not one of the eighteen veintenas properly speaking. It is the five days left over once eighteen twenty day months are subtracted from the 365 day xiuhpohualli, and Nahua tradition treated that leftover pointedly: the days were unlucky, uncounted by name, and best passed in stillness. Sahagun's informants describe ordinary activity curtailed through the five days, travel avoided and loud speech discouraged, out of a fear that whatever befell a person during the Nemontemi would set the pattern for the year to come.
This atlas records Nemontemi twice, deliberately. One entity holds it as a holiday, an observance with its own customs and cautions. This entity holds it as the nineteenth member of the veintena register, the slot the Aztec year sets aside for it, dated, positioned and counted exactly as the eighteen named veintenas around it are. The split mirrors an older one already in the atlas: the Maya Wayeb, the Haab calendar's own five unlucky days, is recorded the same way, once as a calendrical entity and once as an observance, because when this fell and what people did about it are genuinely different questions even when they describe the same five days.
The comparison runs deeper than the shared day count. Both calendars are eighteen month, 365 day solar counts built by cultures with no direct contact, arriving independently at the same solution to the same problem: a year that does not divide evenly into twenty day months. Both answered it the same way, naming eighteen months and setting the remainder apart as dangerous rather than folding it into an ordinary month. Nemontemi and Wayeb are the clearest sign in either calendar that the twenty day month, not the 365 day year, was the unit each culture organized its ritual life around.