The tonalpohualli ("day count") consists of a cycle of 260 days, each day signified by a combination of a number from one to thirteen, and one of the twenty day signs. With each new day, both the number and day sign would be incremented: 1 Crocodile is followed by 2 Wind, 3 House, 4 Lizard, and so forth up to 13 Reed, after which the cycle of numbers would restart (though the twenty day signs had not yet been exhausted) giving 1 Jaguar. The cycle of day signs would continue until 7 Flower, after which it would restart and give 8 Crocodile. It would take a full 260 days (13×20) for the two cycles of twenty day signs and thirteen numbers to realign and repeat the combination 1 Crocodile.
Read more about this topic: Aztec Calendar