The Mexica (Aztecs) used a solar calendar made up of eighteen months, with each month dedicated to a specific god in their pantheon and celebrated with a different set of rituals. Panquetzaliztli, the fifteenth month, dedicated to the national god Huitzilopochtli (Hummingbird on the Left), was significant for its proximity to the winter solstice, and for the fact that it marked the beginning of the season of warfare. In The Fifteenth Month, John F. Schwaller offers a detailed look at how the celebrations of Panquetzaliztli changed over time and what these changes reveal about the history of the Aztecs.

Drawing on a variety of sources, Schwaller deduces that prior to the rise of the Mexica in 1427, an earlier version of the month was dedicated to the god Tezcatlipoca (Smoking Mirror), a war and trickster god. The Mexica shifted the dedication to their god, developed a series of ceremonies - including long-distance running and human sacrifice - that would associate him with the sun, and changed the emphasis of the celebration from warfare alone to a combination of trade and warfare, since merchants played a significant role in Mexica statecraft. Further investigation shows how the resulting festival commemorated several important moments in Mexica history, how it came to include ceremonies associated with the winter solstice, and how it reflected a calendar reform implemented shortly before the arrival of the Spanish.

Focused on one of the most important months in the Mexica year, Schwaller's work marks a new methodology in which traditional sources for Mexica culture, rather than being interrogated for their specific content, are read for their insights into the historical development of the people. Just as Christmas re-creates the historic act of the birth of Jesus for Christians, so, The Fifteenth Month suggests, Panquetzaliztli was a symbolic re-creation of events from Mexica myths and history.