As the second of the nine Yohualteuctin, Itztli governs every ninth date beginning from the second day of the tonalpohualli, a plain rotation running beneath the 260 day count and unconnected to which of the twenty day signs or which trecena a given date falls under. A reader consulting the tonalamatl needed all three registers together, the day sign's own character, the trecena's overall augury as set by its patron pair, and the night lord's turn, to give a complete reading for a birth or an undertaking, and Sahagun's account of the calendar priests, the tonalpouhque, describes exactly this layered consultation rather than any single figure deciding a day's fortune alone.
Itztli's nights were read with the blade's own double character, since obsidian cuts equally for harm and for the letting of blood that Mexica ritual required of penitents and priests as an offering short of full sacrifice, so his presence in a reading carried both a caution and a call to disciplined ritual observance rather than a single fixed omen. Because the night lord cycle of nine shares no common factor with the 260 day tonalpohualli, Itztli's turn falls on a different day sign and a different trecena each time it recurs within a given 260 day round, so no fixed pairing of Itztli with any one day sign existed; the two counts simply interlock and re-align only once every 2340 days, the true period after which every combination of number, day sign and night lord returns to its start.
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