A few facts:
Cinco de Mayo is not Mexican Independence Day. Diez y Seis de Septiembre (Sept. 16th) is Mexican Independence Day.
Cinco de Mayo commemorates the battle of Puebla against the troops of Napolean III.
The celebration of Cinco de Mayo began in the U.S. during the Chicano civil rights / student movements. The battle inspired strength and pride among Chicanos during the movements of the 60's and 70's.