Cinco de Mayo (Spanish for "fifth of May") is a regional holiday in Mexico, primarily celebrated in the state of Puebla, with some limited recognition in other parts of Mexico. The holiday commemorates the Mexican army's unlikely victory over French forces at the Battle of Puebla on May 5, 1862, under the leadership of Mexican General Ignacio Zaragoza Seguín.
The Battle was important for at least two reasons. First, although considerably outnumbered, the Mexicans defeated a much better-equipped French army. "This battle was significant in that the 4,000 Mexican soldiers were greatly outnumbered by the well-equipped French army of 8,000 that had not been defeated for almost 50 years." Second, it was significant because since the Battle of Puebla no country in the Americas has been invaded by an army from another continent.
Cinco de Mayo is not "an obligatory federal holiday" in Mexico, but rather a holiday that can be observed voluntarily. While Cinco de Mayo has limited significance nationwide in Mexico, the date is observed in the United States (also voluntarily) and other locations around the world as a celebration of Mexican heritage and pride. Cinco de Mayo is not Mexico's Independence Day, which actually is September 16, the most important national patriotic holiday in Mexico, nor is it "The Day of the Dead", which occurs on November the 1st and 2nd. (From: Wikipedia, license: CCA-SA)