Like many Mesoamerican calendars, the Tzolk'in, or Sacred Round, calendar operated on a 260-day cycle. One theory for the significance of its cycle length is that the 260 days correlate to pregnancy [source: Maya Mystery School]. Another proposes that the calendar represented the length of time to cultivate corn. However, it's more likely that it was based on the Mayans' reverence for the numbers 13 and 20.
In the Gregorian calendar, we have seven days of the week and, depending on the month, anywhere from 28 to 31 days. The Tzolk'in calendar is made up of a set of 20 day names, symbolized by images called glyphs, and 13 numerals called tones. The days are numbered one through 13, and the names are also given in sequence.
The beginning of the Tzolk'in calendar begins with the first day name, Imix', and the number one. The days continue in sequence, with the second day being a combination of Ik' and the number two; the day names and numbers combine in sequence until all 13 tones are used.
Once the calendar reaches the day 13, denoted by B'en and the number 13, the numbers begin again with one, but the day names move forward with the 14th glyph, Ix. By rotating like this, the two sets form 260 unique combinations of a day name and a number. For instance, once you reach the end of the cycle of day names with 7 Ajaw, the day names begin anew at Imix, and the numerals continue: 8 Imix', 9 Ik', 10 Ak'b'al and so on.
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Think of two interlocking gears, with the 13 numerals spaced around a smaller circular gear that fits inside of the larger gear of day names denoted in hieroglyphics. If you lock those gears together at the number one and the day name Imix', then rotate the gears until you reach one and Imix' again, you'll get 260 unique days. Those gears spin until the final combination clicks into place at 13 Ajaw, marking the end of the year.
It's easy to see the significance the Mayans put in the Tzolk'in calendar. For example, they believed that the date of your birth determines the characteristics you'll show in your personality -- much like some people believe your astrological sign does today.
Holy men also schedule certain events throughout the year based on the Tzolk'in calendar. At the beginning of each uinal (period of 20 days), a shaman would count forward to determine when religious and ceremonial events would occur. He would then select the dates that would be the most prosperous or luckiest for the community.
Despite its myriad functions, the Tzolk'in calendar still couldn't measure a solar year, the time it takes for the sun to make a complete cycle. Because of this, the Mayans needed a more accurate calendar to track the length of time that we regard as a full year.