Sombreros, fakes mustaches, mariachi, tequila, margaritas and lots of tacos, sounds like your typical '5 de Mayo' holiday right? Well, if you grew up in a Mexican family or near Mexican culture that wasn't inside the US, chances are this is just another day.
While Americans may see this day as an opportunity to eat as much guacamole as they can guilt free, the majority of Mexicans just see this reminder of the 'Battle of Puebla' , which took place on May 5th, 1862, as something merely symbolic given that this event happened nearly 50 years after the country's independence (on 1811) and it's relevance is a bit overshadowed by the magnitude Mexico's Independence Day has (now that's a party).
Furthermore, '5 de Mayo' is mostly celebrated in the state of Puebla, and the reason why Mexicans don't celebrate it the way they celebrate Independence, Labour or for that matter Constitution Day is because it hasn't been as imposed by the Government that much beyond or as an official work and school-free holiday. Besides, it actually does have an official recognition as an important cultural holiday in the United States, where the House of Representatives actually passed on an official mandate for partying.
"The House of Representatives recognizes the historical struggle for independence and freedom of the Mexican people and request the President to issue a proclamation recognizing that struggle and calling upon the people of the United States to observe Cinco de Mayo with appropriate ceremonies and activities."
And it's not that Mexicans don't care about their holiday's (believe me, they do) but this date specifically, lacks character in regards with national celebration and it's not festive in general. Yes, '5 de Mayo' is popular for many reasons, whether is because historians see it as a turning point where Mexico's victory was a factor that helped out the federals troops during the U.S. Civil War, because it represents pride and heritage for all of those who were born in Mexico and later on moved to the United States, over the years this holiday has mainly become an American tradition.
In the end, you don't have be Mexican to love and share Mexico's holidays and traditions, and you don't have to be American to indulge in the rich culture, heritage and tradition Mexican-Americans have brought to the United States, but it's important to remember why we celebrate certain things, and the reasons behind it.