Rush just said that Cinco de Mayo is a fake holiday, promoted originally by Corona Beer. It is NOT a holiday in Mexico, just a day for illegal immigrants to guzzle beer ;-)
Historically, it was celebrated only in Puebla to commemorate the famous battle in said city. It was seized on by beer manufacturers (not just Gambinus) and promoted both in the US and Mexico as a new St. Patrick's Day. It caught on first in the US, but has become increasingly popular in Mexico ex-Puebla as well.
Nevertheless, it is not, nor has it ever been a major Mexican holiday a la El Grito de Dolores/Independence Day (September 16).