The nine Lords of the Night, the Yohualteuctin, rotate through every date of the tonalpohualli independently of the twenty day signs and the thirteen numbers that make up the 260 day count, and Xiuhtecuhtli's fixed place at the head of that sequence gives the whole night count its starting point. Each date in the calendar carries three simultaneous readings, a numbered day sign such as 1 Cipactli, a diviner's trecena, and one of the nine night lords, and because nine shares no common factor with 260, the full combination of all three does not return to its first arrangement until 2340 days have passed. Xiuhtecuhtli governs day one of the count, 1 Cipactli, and then recurs every ninth day thereafter, so a reader working the tonalamatl, the painted book of days consulted by the calendar priests Sahagun calls tonalpouhque, would count forward from any known Xiuhtecuhtli day to find the next.
The night lords are usually read as colouring the fortune the day sign and trecena already indicate rather than overriding it, a supplementary voice in the same reading, and Xiuhtecuhtli's presence on a date was generally taken as steadying, since fire at the centre of the count was understood the same way fire at the centre of the house was, as the point everything else is arranged around. The sequence he opens has no equivalent break or reset within the 260 days; it simply turns over continuously beneath the two other counts, a plain arithmetic rotation the priesthood tracked by rote rather than by any myth of its own.
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