I don't think the world is ready for a poly coming-out-day. To be perfectly honest. The groundwork is not laid. Ask a person on the street what they think of poly, and (if they know what the word means at all) they will likely tell you "it's an excuse to cheat." or "Sluts and whores." or "Why bother getting married if you're just going to fool around."
If there is little actual poly-bashing in the world, and if (compared to gays and lesbians) we have suffered little true hate crime leveled against us, it is only because our visibility has been real low and under the radar. That is changing slowly now, and I think that is a good thing, but I think that other events could well be more beneficial before the monogamous majority learn that their neighborhoods are full of scary, culture-threatening nonmonogamists.
In other words, hate speech / hate crime / hate legislation against poly people could very well increase after such an event--- because it puts us on the radar. It will be taken as confrontational and in-your-face. Better to have a period of educational outreach first.
Also because, in my mind, the real purpose and importance of events like these is to educate people that they have a choice. The people who really need activism are the people who were born poly into a traditional monogamous culture, and are miserable and struggling as they attempt in vain to conform, and don't have any conception that there is another way. Let's get the message out to our suffering proto-poly brothers and sisters in the world that "There is Another Way" before we start announcing how many we are.