I've been running into the same issue for that date. At 180 (+10) days out, I just thought that they hadn't loaded the restaurant into the system yet. I had no luck making a reservation on the phone, either. Now, a month later, there's still no availability for any time that day.
I guess we're going back to Raglan Road again (which was my backup reservation). Too bad, I was looking forward to trying a new restaurant.