Admittedly, there were no Mexican-American families in the town in which I grew up, but I don’t remember seeing any widespread national publicity given to Cinco de Mayo during my youth, either. Today, the airwaves are full of commercials for it, and the occasion is used as the pretext for a big party all around the country. We are not the worse for it.
We are a nation of immigrants. As a result, the culture is constantly evolving. We steal shamelessly and joyfully from the traditions of other countries. My favorite example of this is the fact that the Boston Pops always plays the 1812 Overture at the end of its 4th of July concert. What does a piece of music written to celebrate the expulsion of Napoleon from Russia have to do with us? Nothing, and everything.
For a Reactionary, this means it is logically impossible to pick a particular time at which American culture existed in its purest form, before it was defiled by the Other. From what I have read recently, it would appear that the current date of choice is the 1920’s, but you can be certain that the Reactionaries of the day were complaining about all those damn Germans speaking their own language and drinking way too much beer, about southern and eastern European immigrants, and about Irish Catholics. Going back even further, the anti-immigrant Know Nothing Party was created in the 19th century. The process has never stopped, and probably never will, which means the efforts of Reactionaries to save a static idea of America from outsiders is ultimately a lost cause.