This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Two Aztec calendars ran side by side without ever losing a day between them: the tonalpohualli, 260 days long, used for divination and naming, and the xiuhpohualli, 365 days long, tracking the solar year and its festivals. Both cycles began again together only once every fifty two years, the span the Aztecs called a xiuhmolpilli, a bundle of years, tied off like a sheaf of reeds with one reed added for every year that had passed. Toxiuhmolpilia, our years are bound, named the ceremony that marked the join.
On the appointed night, every fire in the Aztec world was put out: temple braziers, household hearths, even the flames tended in royal palaces. Pregnant women were masked, for fear that if the world ended before dawn they would be transformed into wild animals. Children were kept awake and watched, for the same fear reached them. Statues of the gods were smashed and replaced with new ones, and household goods were thrown out to be remade. Nothing of the old bundle of years was meant to survive into the new one unchanged.
The priesthood climbed Huixachtlan, the Hill of the Star southeast of Tenochtitlan, to watch the sky at midnight for the sign that it had not stopped and the new bundle of years could safely begin. A chosen sacrifice was laid on a stone, and a priest drove a wooden fire drill against a hearth board set on the victim's chest until a new flame caught. From that single fire, torchbearers ran in relay to Tenochtitlan's great temple, then out across the city and the wider valley, carrying the new fire to every hearth that had gone dark. By morning the world had crossed safely into its next bundle of years.
The fullest surviving account of the rite comes from Book Seven of the Florentine Codex, the encyclopedic history Bernardino de Sahagun compiled from Nahua elders in the decades after the conquest. The risk the ceremony answered, that the sun might fail to rise again, is recorded there in the informants' own words rather than a friar's paraphrase. Modern archaeoastronomy, following Anthony Aveni's study of Mesoamerican skywatching, reads Toxiuhmolpilia as the mechanism that kept the solar and ritual calendars from drifting apart: two counts of unequal length, resynchronized by main force every fifty two years so that neither one ever ran ahead of the other.
The last New Fire Ceremony held under the Aztec state took place in 1507, the year the Aztecs called Two Reed, fourteen years before Hernan Cortes reached Tenochtitlan. No later performance is recorded; the imperial religion that carried the rite ended with the city it was built to protect.