As the most popular Mexican holiday in the USA, Cinco de Mayo translates to the "5th of May" and is celebrated on that day every year. Make sure you have your facts straight before guests arrive, Cinco de Mayo is not Mexico's Independence Day, which is celebrated September 16th.
Cinco de Mayo is symbolic because, on this date in 1862, a poorly trained and ill-equipped Mexican army of 4,500 soldiers defeated the French invasion of 8,000 well-armed, professionally-trained soldiers in the Mexican town of Puebla.
Hence, "The Battle of Puebla" is a symbol of Mexican unity and patriotism, and a reason for Americans to drink margaritas. Cinco de Mayo is also a good time to console and/or feel sorry for the French.