Setting aside my French heritage in favor of my love for Tex Mex food, and as a public service to all you wonderful readers out there, I offer you the complete Cinco de Mayo party planner.
The fifth of May is a Mexican holiday that celebrates " the Mexican army's unlikely defeat of French forces at the Battle of Puebla". If you are a history buff you can read up on the facts in this Wikipedia article.
I believe the essentials for celebrating Cinco de Mayo are being able to spell it, remembering to mark your calendar, inviting friends over, learning the phrase, "Feliz, y'all", and chowing down on some great Tex Mex food.
2. Tex Mex Food
If you're in Texas, just drive over to your local good quality Mexican restaurant for the fixin's. (That would be The Chaparral in San Antonio, or Matt's in Austin. I told you about Matt's Bob here.)
Everywhere else, just surf on over to Homesick Texan, or Andrea's Recipes and make your own.
3. Tejano Music
What's a party without good music? Here's a PBS clip on the history of Tejano to get you started. Once you know what it is, you can mosey on over to Tejano Radio and listen to Tejano all day!
My Dear Professor has told of many hot South Texas Saturday nights spent hanging out on the porch at the Rock House, listening to the Tejano music echoing over the fields from a little one room wooden Cantina down a narrow caliche road a few miles away.