"In 2011 the Guelaguetza festival will be held on Monday, July 25 and Monday, August 1.
The Guelaguetza is a celebration in which representatives from the many communities of Oaxaca come together and celebrate the diversity of their traditions and cultures. The state of Oaxaca is home to 16 different ethnolinguistic groups and is incredibly diverse. For the Guelaguetza festival, members of these groups gather wearing their traditional clothing and perform folk dances that are particular to their region. At the end of the dancing they throw items to the crowd, products that come from the region they represent.
The Guelaguetza festival as it is celebrated today is a combination of prehispanic celebrations of the corn goddess, Centeotl, and the Catholic feast day of Our Lady of Mount Carmel, which falls on July 16th.
Since colonial times the Guelaguetza festival has been celebrated on the Fortin Hill in Oaxaca (Cerro del Fortin). In the 1970s a special auditorium was built specifically for this celebration, though other events are held there throughout the year. The Guelaguetza Auditorium has seating for 11,000 people. One very special feature of this construction is that it is built into the hill so that spectators looking down at the stage can also appreciate a gorgeous view of the city below.
If you can’t go in July but would like to see a presentation of the Guelaguetza dances, you can attend shows throughout the year at a few different locations in Oaxaca.”
source of the article: http://gomexico.about.com/od/festivalsholidays/p/guelaguetza.htm
source of the pictures: http://www.styde.oaxaca.gob.mx/Guelaguetza2011/galeria_flow/galeria.html