Mictlantecuhtli occupies the fifth and central position in the sequence of night lords, recurring every ninth day beginning from day five of the 260 day tonalpohualli, a placement later commentators have read as fitting for the lord of the underworld to sit at the numeric middle of a nine part rotation, midpoint between Xiuhtecuhtli's opening fire and Tlaloc's closing rain. A date falling under his night was read as carrying a weightier, more dangerous character within the layered system the tonalpouhque worked, tempering or darkening the day sign and trecena's own reading, though Sahagun's text is careful to record that a difficult night lord did not condemn a birth outright, since the trecena's patron and the day sign's own augury carried at least equal weight and a skilled reader balanced all three registers together rather than fixing on one.
Because the nine day night lord cycle and the 260 day tonalpohualli share no common factor, Mictlantecuhtli's turn moves through every day sign and every trecena across the full 2340 day period before the combination repeats, so no single day sign belonged permanently to the death lord; the calendar priests tracked his recurring dates from the tonalamatl tables exactly as they tracked the other eight, a plain administrative reading of a fixed, memorised rotation rather than a special divinatory procedure reserved for him alone.
Sources Cycles of Time and Meaning in the Mexican Books of FateElizabeth Hill Boone with Florentine Codex: General History of the Things of New SpainBernardino de Sahagun; translated by Arthur J. O. Anderson and Charles E. Dibble