Cinco de Mayo is the day of the Mexican Festival of the Grand Chickpea! Oh, wait, no that's not true. It's a minor Mexican holiday celebrating an unlikely win in a battle against French forces in Puebla in 1862. In Mexico people only celebrate it in, where else, Puebla. But in the US it's a big deal, kind of a Mexican Pride Dy even if you aren't Mexican.
I think the reasons for this are obvious. But I'll list them anyway.
1. Americans always like it when the underdog wins, unless the underdog is fighting against the US.
2. If the French had won that battle, they might have gone on to take over all of Mexico afterward, and then we'd have people of French Descent trying to sneak into Arizona. If there's anything Arizonans don't like more than Mexicans, it's French people trying to sneak in to their state and then cut the grass and take care of their kids for less then minimum wage.
If I ever have another daughter I think I will name her Margarita. But that's pretty much not going to happen.