Ersatz holidays

I don't know too much about other countries' holidays, but here in America we seem to have a genius for celebrating the ridiculous.  A case in point is Cinco De Mayo.  Most Americans suppose it is a big Mexican holiday we are helping to celebrate with sombreros and margaritas.  In reality, very few people in Mexico celebrate an obscure victory in that country's losing war against Napoleon the 3rd.  Columbus Day is another ethnic celebration with few takers in Columbus' native Italy.  Why do we do it.   My guess is in the giant melting pot which is American culture, homesick immigrants seized moments in their parent country's history and transferred them to their new homeland in an effort to feel at once part of and distinct from the US.  So we celebrate the banishment of serpents from Ireland on March 17 each year with green beer.   Only in America...