I'm keen on a national coming out day. I'm not actually that keen myself on "poly community" as anything other than a loose network of friends, and I agree that "the poly agenda" is better as a source of humor than something to take seriously. Certainly anything approaching poly culture is deconstructed faster by polys themselves than it can be created.
But what I've seen over the years is that the most radical thing we can do to influence society is simply to say "we're here, and it's working for us". Many people have no awareness that anyone lives this way; others have heard only negatives, like "my cousin tried that, it totally destroyed her marriage". All of this is filtered through societal assumption - the cousin in question may have had a really rich experience and gone on to make radical choices in her life, but these second-hand messages usually get reduced down to "it doesn't work". Many people wind up buying in to concepts like jealousy and possessiveness more than they otherwise would because they think love is supposed to be that way. I've seen so many people react positively and beautifully, just hearing that we're here, and that our approach to relationship has been & continues to be really enriching for us.
So, I think it's a great idea to have a day focused on raising awareness of our existence. It'll be beneficial for us, as society adapts to our presence, but it'll also be beneficial for millions of people who are stuck in relationship models that don't entirely serve them.
As is appropriate for ... ha! I was about to say "our community"! As is appropriate for people as individual as ourselves, I'm not expecting any orthodoxy around this. Some people will think it's a cool idea, and put some effort towards it. Other (probably most) poly people will just ignore it and go on living their lives. That's all fine. It's motivating for me, so I'll probably do some stuff with it here in Minneapolis.