Also resist the urge to wear fake mustaches because it's actually more inconvenient to drink beer that way.
It's worth keeping in mind why Cinco de Mayo is celebrated in the US. The reason is that college-aged Latino-Americans thought it was the most convenient day to celebrate Mexican heritage. Because of that, the point about those who tragically lost their lives (and the fact that it was a battle) seem fairly secondary. You should know the history of Cinco de Mayo*, but I don't think anyone is truly celebrating that history.
* Alternatively, here's what you need to know about the history of cinco de mayo:
[yt]https://www.youtube.com/watch?v=cTZC-j48JWg[/yt]