Reference

Glossary

Tonalpowalli: The count of days: 260 days made by pairing 20 day signs with the numbers 1 through 13.
Xiuhpowalli: The count of the year: 18 veintenas of 20 days, plus the nemontemi, anchored to the observed equinox sunrise.
Tonalli: Day, heat, warmth, destiny — the solar force a day carries and shares with those born in it.
Trecena: A run of thirteen days, named for the day that opens it. The opening sign gives the trecena its character.
Veintena: A twenty-day period of the solar year, each with its own ceremonies.
Nemontemi: The days at the close of the year that belong to no veintena; in leap years the last nemontemi day is extended across two days carrying the same tonalli.
Leap year: Every four years the sun needs an extra day to return to its starting point on the horizon. The count absorbs it by repeating the last day of the nemontemi across two days with the same tonalli — documented by both Sahagún and Durán. Without it, the calendar would drift about 20 days in roughly 83 years.
Xiuhmolpilli: The binding of the years: the 52-year cycle after which the 260- and 365-day counts realign.
Year bearer: The day sign that names a year. Only Tochtli, Akatl, Tekpatl and Kalli can carry one.
Tonalamatl: The book of days — the painted record a day keeper reads.
Correlation: The anchor equating one Mexika day to one day of the Gregorian or Julian calendar.

The twenty day signs