Coatl, the serpent, takes its augury from the animal's own visible habit of shedding its skin and continuing on renewed, and the tonalamatl's readers took the day for movement and exchange rather than for the animal's more common association with venom or danger in later, non Mesoamerican symbolism. Sahagun's fourth book records that 1 Coatl was among the days the pochteca, the long distance merchants whose trading expeditions carried goods and news across Mesoamerica, favoured for setting out on the road, a specific, practical use of the augural calendar that shows the tonalpohualli functioning as working knowledge for merchants rather than only as abstract fate telling for births.
The serpent's renewal symbolism connects this sign to a wider Mesoamerican reverence for the animal that appears far beyond the calendar itself, most visibly in Quetzalcoatl, the feathered serpent, though Coatl as a day sign is not itself identified with that god directly, its patron instead being a water goddess whose own domain, flowing rivers, offered a comparable image of constant motion and renewal. A day of movement and exchange, Coatl's reading fits naturally beside its own trecena, Trecena 9, whose overall augury for merchants and travel the day sign's individual character reinforces rather than complicates.