A couple of Mexican Days -- Cinco de Mayo (5th of May) and Diez y Seis (16th of September). I don't know if the celebrations are anywhere near universal, but in South Texas they are an institution.
There are also many ethnic-oriented festivals around the country, although I don't know specific dates associated therewith.
Then, of course, there's St. Patrick's Day, which I understand is done quite differently here than in Ireland.