Tlaloc holds the ninth and final position among the Lords of the Night, governing every ninth date from day nine of the tonalpohualli onward, his recurrence completing the rotation before it begins again with Xiuhtecuhtli on day ten, so that the full nine lord sequence turns over exactly twenty eight times within the 260 day count with two dates left over, since 260 divided by nine leaves a remainder, a plain feature of the arithmetic that the calendar priests simply tracked from their tonalamatl tables rather than treating as an anomaly requiring explanation.
A date under Tlaloc's night carried, in the tonalpouhque's layered reading, the rain god's association with abundance and with the mountains, a generally favourable colouring for agricultural undertakings though tempered by the same power's capacity for storm and flood that made him a figure of real dread as well as hope. Because the nine day cycle is independent of the twenty day signs and thirteen trecenas, Tlaloc's turn falls across every possible day sign and trecena combination over the full 2340 day period before the three counts together return to their starting alignment, the same mathematics that governs all nine lords equally; his closing position in the list is a matter of the sequence's fixed order as Sahagun records it, not a special final authority over the other eight, who continue their own rotations at the same steady, unbroken pace.
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