Cinco de Mayo is a national, but not federal, holiday in Mexico which is also widely celebrated in the United States. It commemorates the victory of Mexican forces led by General Ignacio Zaragoza Seguin over the French occupational forces in the Battle of Puebla on May 5, 1862.
A common misconception is that Cinco de Mayo is Mexico's Independence Day, which is celebrated on September 16th ("Dieciseis de septiembre" in Spanish). Cinco de Mayo is not celebrated in South America.