That's Nahuatl (Aztec) for "nameless days", referring to the five days at the end of the Mesoamerican calendar that didn't fit the recurring cycles of 13 and 20, leaving a remainder of five that so troubled the astronomically and mathematically-inclined peoples of Precolumbian Mexico that they became the illest of ill-omened days. While the interlocking counts of 260 and 360 days by themselves produce myriad days on which it's best not to get out of bed, let alone start a journey, have a baby, build something, or conduct business in the marketplace, what the Aztecs called nemotemi and the Maya called Wayeb' were a whole different kind of bad -- in the Handbook to Life in the Ancient Maya World (New York: Facts on File, 2002), Lynn Foster writes "During Wayeb, portals between the mortal realm and the Underworld dissolved. No boundaries prevented the ill-intending deities from causing disasters." (thanks Wikipedia!)
I've always felt that the five days between Christmas Day and New Year's Eve had a certain "nameless" feel to them, as they represent the vague and often unsettling liminal period between the end of one year and the beginning of the next. While many people are kept busy during these days by family obligations, travel, and other diversions that may take their mind off of the cosmological flux that such a turning of the calendrical odometer entails, those of us who foolishly wander into work during this lost week find ourselves moving through a strangely quiet and somber simulacrum of our normal workaday lives. There are cars on the road, but there's no traffic to speak of; there are people on the streets, but they never mass into what you'd call a crowd (unless you happen to be at the Returns desk at Best Buy). One almost gets the sense on the morning commute into town that the end of the world is truly nigh, and that maybe not taking these days off in between the holidays wasn't the best use of one's remaining time in this mortal realm.
Then of course, the moment passes: the ball drops in Times Square, everyone blows on noisemakers and sings Auld Lang Syne, and then we promptly spend the first half of the new year accidentally writing the old year atop all of our correspondence and personal checks. Having successfully navigated the nemotemi, we return to the space of regular time, looking forward to that extra Monday off for MLK Day and of course the holiest of holy days in the American calendar, Super Bowl Sunday in early February. While Winter may have only just begun, the new year brings with it the promise of Spring's return and the faint hope of another Summer just beyond the horizon.
Personally I don't find the liminality all that frightening. I actually look forward to this eerie hang-time between past, present, and future, when the people you meet are uncharacteristically reflective about the universe and their place in the grand scheme of things. The Mesoamericans were right to feel that the end of the year is a significant time, but I think instead of locking ourselves indoors for fear of incurring the wrath of nameless forces across the dimensions we should get out there and enjoy these "nameless days". After all, the world only ends once every year, right?