May Day means polling day for us in the UK (I think), it's a riot stepping into one of those porta-cabins and voting for your favourite local candidate (I'll bet).
May Day means people dancing around the May Poler on Blue Peter, brandishing some cloth of some kind and waving it about. And they all have dresses on, and they skip around it, not dance. I think.
I've never heard of Cinco de Mayo though.