Some people have been taught to fear Friday the 13th yet in many indigenous cultures such as the Maya and Mexihca, the number 13 is considered representative of accomplishment, higher consciousness, ancestral memory, and wisdom. Lunar calendars based on 13 moons are vital to a people who are closely linked to nature and who observe important celestial events in order to guide individual and community life.
In Mesoamerican philosophy, there are 13 heavens (thought to also represent 13 levels of consciousness). There are 13 lunar cycles and 13 major joints in the human body. Men and women are considered to have reached elder hood at age 52, which is the completion of a cycle of four rounds of 13 years each. The 260-day Tonalpohualli (Aztec) divinatory calendar consists of 20 groups of 13-day periods called treceñas which are combined with a series of day signs (Rabbit, Wind, Deer, Rain, etc.). The 13 treceña is associated with the venerable Tlazolteotl, an aspect of the earth mother.
Take the breathtaking ceremony undertaken by the Voladores de Papantla, wherein the voladores or "fliers," sometimes called hombres pajaro, birdmen, launch themselves from the top of a pole of up to 150 feet in height, and slowly descend circling the pole. In their descent, each volador circles the pole 13 times — thirteen times for each of the four voladores, for a total of 52 rotations, representing the number of years in the Mesoamerican calendar cycle. This incredibly beautiful ritual is a part of the rich cultural heritage of Mexico and was recognized as such by UNESCO on their Representative List of the Intangible Cultural Heritage of Humanityin 2009. See, Voladores de Papantla, Los Voladores (in Spanish) (Click on the images below for more information.)
The number 13 is also considered to have associations with the feminine in cultures with a lunar calendar as well as with menstruation, and turtle.
If one looks closely at the shell of turtle, one can see that it is divided into thirteen large central plates (scutes) which represent the thirteen moons of the lunar year — if you count the smaller segments around the lower edge of the shell, you will find there are 28, which represents the 28 days between new moons. Turtle also has spiritual association to the ocean, water, and creation. A woman, through her menstrual cycle, goes through the different lunar energies (new moon, crescent moon, full moon, waning moon). These in turn are connected to the different seasons (spring, summer, autumn and winter). Hence the profound connection of women with Grandmother Moon. "After the Sky Woman placed earth on the turtle’s back, she danced around the turtle 13 times. Each time the turtle grew until it became the entire land. Turtles, the Akwesasne [Mohawk] note, have 13 squares on their backs that represent the 13 times the Sky Woman danced around counter-clockwise. — Source
"The Hopi sun priests make use of thirteen points on the horizon for the determination of ceremonial dates.”— Source
Read, "Thirteen Moons on a Turtle's Back" by Joseph Bruchac and Jonathan London.
"In Native American legend, the thirteen scales on Old Turtle's back hold the key to the thirteen cycles of the moon and the changing seasons. These lyrical poems and striking paintings celebrate the wonder of the seasons, from the Northern Cheyenne's Moon of the Popping Trees to the Big Moon of the Abenaki." (Available on Amazon)
"The turtle’s back is a significant symbol used in Iroquoian media. It represents the creation of Turtle Island also known as the continent of North America, on turtle’s back or more accurately the “Earth Grasper”, from our Creation Story. A closer look will reveal a pattern of thirteen individual segments on his back which represent each moon of the Iroquoian cycle of seasons. These segments are named according to seasonal practices and natural environmental occurances e.g. Green Corn, Harvest etc. Further to this there are twenty-eight platelets which form an edging around the shell. This is the number of days in the moon cycle." —Source
Read more on the Aztec calendar, A Mexican Count of Days - Xiwitl Chikome Akatl by Kurly Tlapoyawa