The Aztec civil year was not divided into weeks and months like those of today, but into periods of 20 days each. Eighteen of these periods made up the calendar year, which included five ‘nothing’ days to bring the count correctly up to 365 days. There was an extraordinary system of working out a divinatory year, which consisted of periods of 260 (20 x 13) days, within which all the normal variations of fate took place. When the magical and civil calendars came into line, which occurred every 52 years, when the star group Pleiades ((The Seven Sisters) reached the center of the sky, there was a great festival at which new fire was ceremonially made and distributed, and every temple was renovated as if creation had begun again.
The Aztec day began at sunset. There was no calculation of hours, because in all normal life it was quite sufficient to observe sunset and sunrise. There was a period when the sun was at its highest point in the day, at noon, and at night there was midnight. This was probably determined by noting the stars which appeared in the zenith when the sun set, and waiting till they were setting on the western horizon. This procedure would be sufficiently accurate for the Tlatoani, the Speaker, and the priests to make the midnight observation of the starry sky.
There were nine gods who were known as the Lords of the Night. Each in turn held sway over the hours of darkness for a night. There were 13 Lords of the Day, and each of them held sway in turn over a day. Thus, in the religious calendar of 260 days, which consisted of the combinations of 13 numerals with the 20 days of each period, there was this other rhythm of ruling deities of each period. Each 13 -day period had a protecting pair of deities, and similarly the 20-day periods had each a guardian spirit. Within this fantastic almanac there were also 13 magical birds flying with their individual influences on fate.