Tonalli has three meanings: day, heat of the sun, and luck. 
Tonalli as day
A day in the Anahuaca calendar is a combination of a number, 1-13 and a day sign, Cipactli-Xochitl. There are 260 unique day signs because 260 is the resulting product when 13 is multiplied by 20. This 260 day cycle was used for divination in Pre-Cuauhtemoc times. Each day sign was considered to be good, bad, indifferent, or both good and bad depending on the various influences of the numbers and days. The number 13 for example was considered to be the luckiest and most powerful day whereas a day sign like malinalli was bad because it is associated with beasts. Many of the surviving Pre-Cuauhtemoc codices contain a complete tonalamatl (book of days) which were consulted by a Tonalpouque (interpreter of day signs)on important days such as childbirth and marriage.
- Nahuatl-English English-Nahuatl, Fermin Herrera, page 196.