The original page 13 of the Codex Borbonicus, showing the 13th trecena of the Aztec sacred calendar. This 13th trecena was under the auspices of the goddess Tlazolteotl, who is shown on the upper left wearing a flayed skin, giving birth to Cinteotl. The 13 day-signs of this trecena, starting with 1 Earthquake, 2 Flint/Knife, 3 Rain, etc., are shown on the bottom row and the column along the right side.
A trecena is a 13-day period used in pre-ColumbianMesoamerican calendars. The 260-day calendar (the tonalpohualli) was divided into 20 trecenas. Trecena is derived from the Spanish chroniclers and translates to "a group of thirteen" in the same way that a dozen (or in Spanish docena) relates to the number twelve. It is associated with the Aztecs, but is called different names in the calendars of the Maya, Zapotec, Mixtec, and others of the region.
Many surviving Mesoamerican codices, such as Codex Borbonicus, are divinitory calendars, based on the 260-day year, with each page representing one trecena.