The Aztecs used two different calenders,
one measured time, while the other was used to fix religious
festivals. The time-measuring calender was used to fix the best
time for planting crops, while the religious calender told when
to consult the gods. In the time-measuring calender, one year
had 365 days divided into 18 months. Each month had 20 days,
and there were 5 extra days at the end of the year, which were
thought to be bad-luck days when disasters were most likely to
happen. The fundamental Aztec calender, the religious calender,
was a 260-day cycle, called the tonalpohualli, or the "count
Elizabeth Hill Boone excellently described
how the religious calender worked in her book, "The Aztec
World." She said, "Twenty day signs ran consecutively,
from crocodile through flower, repeating after the 20th day.
Beside the day signs ran 13 day numbers, 1 through 13, the numbers
advancing with each day up to 13, when they repeated again with
1. Thus, the day count began with 1 Crocodile, 2 Wind, 3 House,
4 Lizard, and continued up to 13 Reed, when the numbers began
again with 1: 1 Jaguar, 2 Eagle, 3 Vulture, and so forth. The
20 day signs and the 13 numbers, advancing side by side, yielded
260 uniquely named days. "
Both calenders ran together and the
same day in each fell at the same time once every 52 years. Thus,
Aztec time was divided into 52-year cycles, similar to our current
100-year cycles called centuries. Different days belonged to
different gods, so days could be good or bad depending on which
god's day it was. A child born on a bad day received its name
on a good day, to rule out all harmful effects of the bad day.
At the center of the Aztecs' calendar
stone is the sun god, Tonatiuh. He is surrounded by symbols of
the five world creations. The symbols of the 20 days of the solar
month are depicted on the stone. Also, eclipses of the sun were
foretold by the calendar stone.