It's sad to say that when some people think about Cinco De Mayo, they think about an excuse to ram as many bottles of Corona into their orifices as they can and maybe eat a jar of salsa, if they remember to. But the 5th of May is actually one of the most important days in Mexican history, and if you want to be a culturally sensitive dude who gets laid a lot, you should be able to talk the talk and walk the walk. So here we go.
Many people labor under the misconception that Cinco De Mayo is the "Mexican Fourth Of July." This is completely false - in Mexico, they shoot fireworks off pretty much every day, so every day is the Mexican Fourth Of July. In addition, "cinco" means five, not four. A rookie mistake. Instead, Cinco De Mayo celebrates the anniversary of the Battle of Puebla, in which Mexico repulsed the snooty French army in 1862.
The Battle of Puebla was fought around an issue that many modern people can relate to: inability to pay credit card debt. Mexican President Benito Juarez decided to up and quit paying interest on the country's national debt one day (because he needed money for whatever the 1800s equivalent of an Xbox 360 is, possibly a goat with colorfully painted fur). This incensed the moneylenders, and they sailed the seas to pop a musket cap in his hindquarters. At the Battle of Puebla, 4,000 Mexican soldiers defeated 8,000 French soldiers, giving us a simple mathematical formula to determine how much better Mexican people are than French people. It marks the last time a country in the Americas has been invaded by an enemy from another continent.
Although the word "Mayo" is actually in the name of the holiday, mayonnaise is not consumed on May 5th for patriotic reasons. The pronunciation is closer to "my-oh," rhyming with former Tonight Show sidekick Ed McMahon's trademark "Hey-ohhhhh" exclamation. If you go to a Cinco De Mayo party bearing jars of Hellman's, you'd best be ready to drink it all yourself.
In Mexico, Cinco De Mayo isn't really such a big deal, taking second to holidays like Mexican Independence Day, Halloween and Earth Day. But in America, where we really like to act like we know what's up, it's become an all-purpose way for us to say "Brown hard-working neighbor to the south, we get you." Except for Arizona, which outlawed the holiday in 1971 under penalty of death.