Cinco De Mayo, the 5th of May 1862 was a victory for the Mexican Army over the French occupation of Mexico. It is not the Mexican Independence Day, that happens September 16th. Cinco De Mayo is celebrated for the victory of the Battle of Puebla. It was huge victory in the war but did not win the war outright. It's victory was symbolic and moral boost because of it's outright disadvantage to the Mexican Army facing a much stronger French Army.
Before the Battle of Puebla Mexico was engaged in a civil war with an eerily similar divide of the people. A war of conservatives and liberals who fought over the separation of church and state in government. This war bankrupt Mexico and the president decided to quit paying debts to European nations as a result. So Euro's got pretty pissed and the especially testy Napoleon III decided to invade and make Mexico his own land. The Mexicans weren't having it. They kicked some ass at the Battle of Puebla but ultimately that jerk Napoleon just sent even more troops and within a year had taken Mexico City. However the Frenchies didn't make it very long as rulers because us badass' to the north decided to free our slaves and stop fighting each other. The Americans then focused their testosterone on the French in Mexico and offered our neighbors to the south some much needed support. The French weanies got scared and ran home fearing a war with us. Smart move on their part. We didn't ultimately help aside from puffing our chests at the French. The Mexican guerilla armies did the fighting and killed Maximillian the French ruler and his comrades.
So you see, Cinco De Mayo is a symbolic source of pride for Mexican people. It also is a bit embarrassing for the French and is neither here nor there for the Americans. But the scary thing is how similar the events leading into these clashes are to our own current political climate. If history has a way of repeating itself we may just be calling on our brothers to the south for a little bailout at some point too... And it's all the more reason why Americans really have no reason to be overly celebratory today other than a misguided excuse to get loaded. Which is fine with me honestly. I love Mexico and it's people and I don't mind sharing in their pride, but let's be real about it... And even more importantly let's understand how important our neighbors are to our own culture and stop this bullshit about giant walls and rapists and drug dealers. Because the reality is Americans will consume enough drugs celebrating Mexico's holidays that we'll keep them at war for many years to come. It's a happy face conundrum.