The 260-day Sacred Round (also known as the Tzolk’in or Tonalpohualli) is the heart of the Aztec Calendar, the Mayan calendar and nearly every native calendar of Mesoamerica. The Sacred Round is composed of two cycles, one of 20 named days, and one of 13 numbered days, each of which continuously repeats. It takes 260 days for the two cycles to realign on the beginning day of each cycle (260 = 20×13), so the Sacred Round has 260 days.
The 20-day Veintena
The fundamental cycle is the 20-day count, called the veintena. That is Spanish for a “group of twenty” even as docena, reminiscent of “dozen”, is Spanish for a group of twelve. Similarly, trecena is a group of thirteen.
Each of the days is named and has an associated picture or glyph. The 20 day glyphs are shown in a circle in the famous Aztec Calendar Stone.
The 20 day glyphs are named (with Mayan names in parantheses):
Crocodile (Imix)
Wind (Ik)
House (Akbal)
Lizard (Kan)
Serpent (Chicchan)
Death (Kimi)
Deer (Manik)
Rabbit (Lamat)
Water (Muluk)
Dog (Ok)
Monkey (Chuwen)
Grass (Eb)
Reed (Ben)
Jaguar (Ix)
Eagle (Men)
Vulture (Kib)
Movement (Kaban)
Flint (Etznab)
Rain (Kawak)
Flower (Ajaw)
The days were said by the Mayans to represent the steps in a person’s life (from before birth to after resurrection). Thus, after the first day (Crocodile) which represents conception, the second (Wind) is quickening, when the spirit enters the baby still in the womb, and the third (House) is the day of birth. The fourth (Lizard) is the day when evil is first allowed into the child’s life, and the fifth (Serpent) represents repenting of evil ways and returning to the good life. The sixth (Death) represents death, and the seventh (Deer) represents the fleeting journey into the spirit world. The final thirteen steps are less clear, as is our understanding of life after death.
Note that the circular arrangement is important because there are several pairs of opposites. For example, the fifth day in the cycle is “Serpent,” and directly opposite the serpent is the “Eagle.” The eagle/serpent pair was so important to the Aztecs that their capital (now Mexico City) was founded where they saw the eagle with the serpent in its talons (now immortalized on the Mexican flag.)
The veintena is also the fundamental cycle of the Mayan Long Count, which is not discussed here. Moreover, Native American astrology, which assigns similar traits to people, is based on the day of the veintena on which they are born.
The 13-day Trecena
The 13-day cycle, called the trecena, simply numbers the days rather than using pictures. That is, the days are numbered from 1 to 13 and then that cycle repeats indefinitely. The two cycles continue separately from each other. It is very similar to how there are seven days in a week, and they just keep repeating. No extra days are ever inserted into the week, and none are ever inserted into either the trecena or the veintena.
The two cycles each separately move through one day at a time, so the effect is similar to two gears, one with 13 cogs and the other with 20, as shown in the illustration. Thus, the Sacred Round begins at the position shown on the day 1 Crocodile. The next day is 2 Wind and then 3 House. That may sound confusing if were expecting 1 Crocodile to be followed by 2 Crocodile and then 3 Crocodile. But we do the same thing on our modern Gregorian calendar. The day after Monday the 1st (of a month) is Tuesday the 2nd. That is because the week of seven days progresses daily even as do the days of the month. The thirteenth day of the cycle is 13 Reed. The following day is 1 Jaguar, because the trecena starts over, but the veintena continues on through all 20 figures. It takes 260 days to get back to the starting position (13 x 20 = 260), so the Sacred Round contains 260 days.
One can ask whether the Sacred Round consists of 13 cycles of 20 days or 20 cycles of 13 days. Most Native Americans think of the Sacred Round as 20 trecenas. There are two conventions for naming the twenty trecenas. The Aztecs named them for the first day, that is, for when the day number was 1. Thus the first trecena was called “1 Crocodile” after the first day. The second trecena was “1 Jaguar”. (See Aztec ordering Table using both written names and modern icons.) Thus the first trecena is named 1 Crocodile, for its first day. The second is 1 Jaguar. A Native American example of the thirteenth trecena “1 Movement” is illustrated in a wikipedia image).
In passing, we can note that almost every author illustrates this entire table backwards and shows the Sacred Round as 13 veintenas rather than 20 trecenas. The reason they do so is that it is much easier to make a table of 260 numbers rather than 260 words or pictures. But it is important to get it right if one actually wants to use it.
The Maya and Western calendars are correlated by using a Julian day number (JDN) of the starting date of the current creation — 13.0.0.0.0, 4 Ajaw, 8 Kumkʼu. This is referred to as a “correlation constant”. The generally accepted correlation constant is the Modified Thompson 2, “Goodman–Martinez–Thompson“, or GMT correlation of 584,283 days. Using the GMT correlation, the current creation started on September 6, −3113 (Julian astronomical) – August 11, 3114 BCE in the Proleptic Gregorian calendar. The study of correlating the Maya and western calendar is referred to as the correlation question. The GMT correlation is also called the 11.16 correlation. The GMT correlation is used in my work on the Sacred Round.