The Mexican year was a cycle of 365 days, without any intercalary addition or other correction. In course of time it almost lost its seasonal significance because of the omission of the extra hours included in the solar year, and furthermore many of its festivals and occasions were altered by high-priests and rulers to suit their convenience. The Mexican nexiuhilpililztli (binding of years) contained fifty-two years, and ran in two separate cycles--one of fifty-two years of 365 days each, and another of seventy-three groups of 260 days each. The first was of course the solar year, and embraced eighteen periods of twenty days each, called "months" by the old Spanish chroniclers, with five nemontemi (unlucky days) over and above. These days were not intercalated, but were included in the year, and merely overflowed the division of the year into periods of twenty days. The cycle of seventy-three groups of 260 days, subdivided into groups of thirteen days, was called the "birth-cycle."
People in a barbarous condition almost invariably reckon time by the period between the waxing and waning of the moon as distinct from the entire passage of a lunar revolution, and this period of twenty days will be found to be the basis in the time-reckoning of the Mexicans, who designated it cempohualli. Each day included in it was denoted by a sign, as "house," "snake," "wind," and so forth. Each cempohualli was subdivided into four periods of five days each, sometimes alluded to as "weeks" by the early Spanish writers, and these were known by the sign of their middle or third day. These day-names ran on without reference to the length of the year. The year itself was designated by the name of the middle day of the week in which it began. Out of twenty day-names in the Mexican "month" it was inevitable that the four calli (house), tochtli (rabbit), acatl (reed), and tecpatl (flint) should always recur in sequence because of the incidence of these days in the Mexican solar year. Four years made up a year of the sun. During the nemontemi (unlucky days) no work was done, as they were regarded as ominous and unwholesome.
We have seen that the civil year permitted the day-names to run on continuously from one year to another. The ecclesiastical authorities, however, had a reckoning of their own, and made the year begin always on the first day of their calendar, no matter what sign denominated that day in the civil system.