Guelaguetza starts on Monday. A major pre-hispanic festival in the southern state of Oaxaca, Wikipedia has quite a lot to say about it:

The Guelaguetza, or Los lunes del cerro (Mondays on the Hill) is an annual indigenous cultural event in Mexico that takes place in the city of Oaxaca, capital of the state of Oaxaca, as well as in nearby villages. The celebration centers on traditional dancing in costume in groups, often gender-separated groups, as is traditional, and includes parades complete with indigenous walking bands, native food, and statewide artisanal crafts such as prehispanic-style textiles. Each costume (traje) and dance usually has a local indigenous historical and cultural meaning. Although the celebration is now an important tourist attraction, it also retains deep cultural importance for the peoples of the state and is important for the continuing survival of these cultures.

Oaxaca has a large native indigenous population, well over 50 percent of the population, compared to 20 percent for Mexico as a whole (depending on systems of classification). Indigenous culture in Oaxaca remains strong, with over 300,000 people in the state who are monolingual in a wide variety of native indigenous languages and many others who are bilingual in Spanish, or follow a predominantly indigenous lifestyle. Unlike nearby Yucatán also located in the Mexican Southeast, where the indigenous culture consists of closely related groups of the same culture (Mayans), the indigenous people in Oaxaca are from many different cultures. Zapotec and Mixtec are the two biggest ethnic groups in terms of population and area, but there are also a great number of other groups, and all have their own unique traditions and speak diverse, mutually unintelligible languages. The Guelaguetza celebration dates back long before the arrival of the Spanish and remains a defining characteristic of Oaxacan culture.[1] Its origins and traditions come from prehispanic earth-based religious celebrations related to the worship of corn and the corn god.[2] In contemporary Oaxaca, indigenous communities from within the state gather at the Guelaguetza to present their native culture, mainly in the form of music, costumes, dances, and food. It is the most famous indigenous gathering of its kind in Mexico.[2]Like many indigenous traditions in Mexico, this festival was adapted to and mixed with Christian traditions after the Spanish conquest of the area. The human sacrifice of a virgin slave girl[citation needed] was eliminated from the event, and the Guelaguetza instead became mixed into a celebration honoring Our Lady of Mount Carmel (Virgen del Carmen), emphasizing marianism combined with the surviving beliefs. In the early part of the 20th century, after a severe earthquake in the 1920s that destroyed most of the city, the festival was re-organized as a statewide cultural event to rebuild the morale of the peoples of Oaxaca “La Guelaguetza de la Raza”.[2] It began to take on a more modern form as a display of each peoples/region’s unique dance, and also started to become more of a show than a spontaneous festival. In the 1970s a stadium dedicated to the Guelaguetza was built on a prominent place on Fortin Hill in the center of the city. National and international tourism became increasingly popular when the ancient city of Oaxaca became a UNESCO world heritage city in 1987 and when a modern limited access highway was built to the city in November 1994. Before the highway, transportation was so slow that it was virtually impossible to journey through the rugged, often remote, mountainous high-altitude terrain to reach Oaxaca City from other cities such as Mexico City for a weekend trip to the Guelaguetza.

The celebration takes place on consecutive Mondays at the end of July in towns around the state and in the capital city’s open-air amphitheater built into the “Cerro del Fortín”, a hill that overlooks central Oaxaca City. The word Guelaguetza comes from the Zapotec language and is usually interpreted as the “reciprocal exchanges of gifts and services” in keeping with the importance in indigenous cultures of sharing, reciprocity, and extended community.[1] The Guelaguetza celebration also includes many other side events, including a performance of “Princess Donaji”, an epic prehispanic theatrical presentation performed the day before the Guelaguetza itself begins.