I'm sorry to sound like a dick, but I'm gonna let my asshole side out for a moment here.
You don't just plan an event and then decide to run it on the same day. You set it up at least several days in advance, so people have a chance to see it and plan around it. What if someone would have really wanted to participate, but didn't see the thread in the 3 HOUR WINDOW you've given? What if someone was busy tonight, but could have easily made room if given the opportunity?
I hope I see another one of these in the near future, maybe one that's actually planned well.
I'm having a shitty day today and this is my way of venting I suppose.
Carry on.