@farmerman,
here are you do and do nots for today:
1. 'Happy Cinco de Mayo!'... said no Mexican ever
Today is not Mexico's independence day (which is September 16) so you should refrain from treating it as such. You will rarely see a Mexican in the United States or in Mexico -- outside of where the battle actually happened -- acknowledge, let alone celebrate, the holiday. So simmer down.
2. Resist the urge to wear sombreros and fake mustaches because, well, stereotypes
While it may seem like all fun and games, these costumes essentially reduce cultural and traditional costumes to a caricature. Take it from the Penn State sorority sisters who learned the hard way after donning ponchos during one of their "Mexican-themed" parties. (Not to mention the offensive signs they made to accompany their costumes.)
3. Enjoy traditional Mexican libations, but don't use Cinco de Mayo as an excuse to get brutally inebriated
There's a thin line between enjoying a drink special with friends and running amok after taking 13 shots of tequila. Plus, blacking out drunk is sort of disrespectful given Cinco de Mayo is in remembrance of a battle, meaning lives were lost. In other words, keep it classy, folks.
4. Avoid using your high school level Spanish with native Spanish speakers
Any attempts to awkwardly "connect" will likely come off as cheesy and empty. And, please don't ask anyone to "speak Spanish" for your amusement because that's just disrespectful. Plus, not all Latinos speak Spanish so you may be striking another nerve.
5. Don't plan a trip to Mexico to celebrate a 'real' Cinco de Mayo
Outside of the state of Puebla in Mexico, the holiday isn't really celebrated at all. Even the celebrations in Puebla are much more tempered and family oriented. If you're genuinely interested in Mexico, head to San Diego, San Antonio or even Mexico City on September 16 and indulge in an authentic experience in all things Mexican.