If either the UCAS or the CAS consider themselves a continuation of the USA, then they will certainly celebrate the event, with fireworks and beer, and more fireworks. Basically the UCAS was a merger of a majority of the USA and of Canada. Obviously the people who still sees themselves as Americans will likely celebrate. Those who see themselves as Canadian will likely just let it be another day, no big deal. For the folks in between, they will probably celebrate this as an important milestone. (The fact that the UCAS has a president seems to indicate that the country as a whole leans more to the American side, than to the Canadian side of things. So I would predict some celebration going on.
As for the CAS, these guys claim to be American, and not just any American, but Confederates. Since they broke away from the UCAS to be more American than the UCAS, I definitely see them having a huge party with lots of fireworks, and maybe even a couple of riots that end up attacking any UCAS property they can their hands on.
I think the other nations that came from the U.S. have distance themselves and have decided to do their own thing, and therefore won't celebrate. It is highly possibly that anyone 50+ who was actually born a United States citizen may celebrate, or at least spend the time thinking back to the good ol' days.