Here is a bit of history about Cinco de Mayo, from Wikipedia:

Cinco de Mayo (Spanish for "5th of May") is a regional holiday in Mexico, primarily celebrated in the state of Puebla. It is not an obligatory [Mexican] federal holiday.^{[1][2][3][4][5]} The holiday commemorates an initial victory of Mexican forces led by General Ignacio Zaragoza Seguín over French forces in the Battle of Puebla on May 5, 1862. The date is observed in the United States and other locations around the world as a celebration of Mexican heritage and pride.

A common misconception in the United States is that Cinco de Mayo is Mexico's Independence Day; Mexico's Independence Day [from Spain] is actually September 16 (dieciséis de septiembre in Spanish), which is the most important national patriotic holiday in Mexico.