Literally "the Fifth of May," Cinco de Mayo is a Mexican Holiday celebrating the Battle of Puebla, which took place on May 5, 1862. In 1861, France sent a massive army to invade Mexico, as they wanted to collect on some war debts. The French army was much larger, better trained and equipped than the Mexicans struggling to defend the road to Mexico City.
It rolled through Mexico until it reached Puebla, where the Mexicans made a valiant stand, and, against all logic, won a huge victory. It was short-lived, as the French army regrouped and continued; eventually taking Mexico City, but the euphoria of an unlikely victory against overwhelming odds is remembered every May fifth.