Veinte de Agosto Park is an acre of grass & trees in the heart of downtown Tucson at the point where Congress & Broadway split into one-way streets. "Veinte de Agosto" is Spanish for 20th of August.
In late 2011, the park became the tent camp of Occupy Tucson protesters.
The Tucson Visitors & Convention Bureau website claims that Veinte de Agosto Park celebrates the birth-date of Tucson, August 20, 1775.
Wikipedia has a different take. "Veinte de Agosto is the national holiday celebrating teachers and mentors across Mexico. Although the name would indicate the holiday is held on August 20, it is actually celebrated according to the lunar calendar. The first celebration of this glorious occasion fell on Friday, August 20, 1974. Every 6 years, when the holiday falls on a Friday again, leads to an even greater celebration than normal."
Veinte de Agosto Park is also the site of a statue of a mounted Pancho Villa, famous Mexican revolutionary general. Additionally, six flags fly over the park representing the six governmental administrations that ruled here. However, there were actually seven: Spain (1775-1821); Mexico (1821-1853); United States of America (1853-1861); Confederate Arizona Territory (1861-1862); U.S. New Mexico Territory (1862-1863); U.S. Arizona Territory (1863-1912); U.S. State of Arizona (1912- ).