Why on earth do American celebrate Cinco de Mayo?

In speaking with many Mexican friends, they all say that it's about
their equivalent of Labor Day. If we wanted to have a party about
Mexico, here in the US, shouldn't we celebrate "El Grito", the 16th of
September? Mexican Independence Day?

Simplistic thinking says: "it started by Corona Beer's
advertisements". Yet, I'm sure there's more to it than that.

The obvious that it's a good day to have a beer or margarita is also,
not much help. Well . . . it never hurts to have a beer . . . but
that's another story.

Anybody out there know anything?