A few common misconceptions...
Cinco de Mayo is Mexican Independence Day, right? No, little grasshopper. In fact, Mexican Independence is celebrated with big parties throughout the country in the evening of September 15 and different parades during the day on September 16. It is the most important holiday in Mexico.
It's still a big, official holiday though? Still no. It is an annual commemoration of a single Mexican victory over the French during a battle in Puebla as part of the 1862 French invasion. The invasion culminated with Napoleon appointing Maximilian I as Emperor of Mexico (Mexico’s second and last Empire). In Mexico, May 5th is a regular business day with a couple of commemorative parades in Mexico City and Puebla, but not much more than that.
Wow. How did Cinco de Mayo get to be such a big deal here then? The history behind the US festivities comes from Mexicans in California strongly resisting French invasion and celebrating the victory. The holiday gained popularity in U.S. regions with strong Mexican representation. Years later, beer companies saw it as a big marketing opportunity. Currently, L.A. hosts the largest Cinco de Mayo celebration in the world. So now you know that if you want to celebrate a real Mexican holiday do so by drinking some Tequila in September!