While not to be confused with the Mexican Independence day that falls on September 16, Cinco De Mayo is a day of great importance to the Mexican and Chicano communities because it marks the victory of the Mexican Army over the French at the Battle of Puebla(Batalla de Puebla).
Although the Mexican Army was eventually defeated, everything about the day came to represent a symbol of Mexican unity and patriotism.
The holiday of Cinco de Mayo is primarily a regional holiday in Mexico. There is some limited recognition of the holiday in other parts of the country. However, in Mexico, Cinco de Mayo has a whole different significance the it does in the United States. In ways, there it's sort of like our 4th of July.
In the United States, we don't celebrate it because of the Mexican's victorious battle the holiday was founded upon, but of the Mexican culture as a whole. No matter what decent we may come from, we all stop and celebrate Cinco de Mayo because it recognizes one individual culture and their beliefs.
Granted, it has become over commercialized like a lot of the other holidays in the U.S., but the principle of the whole thing is still there and I think that is whats most important!
Happy Cinco de Mayo Everyone!