The Aztecs devoted two twenty-day months to festivals for the dead.
The Aztec solar calendar, called the xiuhpōhualli, contained eighteen periods of twenty days, commonly known today as veintenas. Two consecutive veintenas were associated especially with the dead: Miccailhuitontli, the smaller feast of the dead, and Huey Miccailhuitl, the greater feast of the dead.
These observances occurred in the late-summer portion of the ritual year and involved ceremonies, offerings, flowers, music, food, and public religious activity. The festivals were dedicated within a broader calendar system in which every twenty-day period had its own religious and agricultural significance.
The answer is sometimes misunderstood because “months” is a modern shorthand. The Aztec periods were not lunar months; each contained exactly twenty days, and the year also included five additional nameless days. Spanish chroniclers recorded the festivals, but their dates and descriptions can vary because the calendar’s correlation with the European calendar is debated. The two dead-focused periods nevertheless explain why Day of the Dead is often discussed alongside earlier Mexica traditions.