Although Cinco de Mayo has become a predominantly American holiday, most people don't know why we celebrate it. Cinco de Mayo is only celebrated regionally in Mexico; in and around the state of Puebla. During the early-mid 19th century, parts of Mexico were occupied by France. After (what turned out to be) a series of civil wars, the splintered Mexican government stopped paying foreign debts, causing tension between various European countries. As a result, England, Spain, and France arrived on the shores of Veracruz to demand reimbursement. Both England and Spain were able to reach settlements, but France remained in occupation. On the 5th of May, 1862, Mexican forces defeated the French army in an unlikely victory at the Battle of Puebla. Cinco de Mayo is not to be confused with Mexican Independence Day, which is widely celebrated in Mexico on September 16th.
In honor of Cinco de Mayo, I've posted a video of Charlie Rose's interview with three of the best contemporary Mexican filmmakers: Guillermo del Toro, Alejandro Gonzalez Inarritu, and Alfonso Cuaron: