Feliz Cinco de Mayo

What IS Cinco de Mayo exactly? I did a google search (because although I did grow up in California I've forgotten what it was but I knew it wasn't Mexican Independence Day) and what the 5th of May celebrates is the defeat of the French at the Battle of Puebla, which was led by Gen. Zaragosa. Of course, Cinco de Mayo, besides being a school holiday in California was celebrated as a big drinking holiday--the Mexican equivalent of St. Patrick's Day. In fact, the analogy seems all too apt, because what both holidays have in common is a celebration of a little known ethnic/national event or person, a holiday which is more renown in the U.S. among Irish Americans and Mexican Americans than among Irish in Ireland and Mexicans in Mexico. And, of course, for the non-Mexican and non-Irish in the U.S., it's a day to drink. Weird.