Cinco de Mayo (Spanish for “fifth of May”) is celebrated in the United States and in Mexico, primarily in the state of Puebla, where the holiday is called El Día de la Batalla de Puebla (English: The Day of the Battle of Puebla).
It originated with Mexican-American communities in the American West in the 1860s as a way to commemorate the cause of freedom and democracy during the first years of the American Civil War, and today the date is observed in the United States as a celebration of Mexican heritage and pride.
In the state of Puebla, the date is observed to commemorate the Mexican army’s unlikely victory over French forces at the Battle of Puebla on May 5, 1862, under the leadership of General Ignacio Zaragoza Seguín.
In the United States, Cinco de Mayo is generally mistaken to be Mexico’s Independence Day but it is not. That most important national holiday in Mexico is celebrated on September 16. (Sounds like another opportunity to celebrate with Margaritas to me!)
For more, check out Wikipedia.