We did this recently. You'll probably want to get to the lounge early, as it can get busy. We got there sometime between 5:30 and 6 pm and there was only one table left in the lounge. You can come back to the viewing deck to watch the fireworks. For us, it took a while to get our food. By the time we were done, it was around 7:30. The elevator operator recommended that we return to the viewing deck by 8:30. We just walked around the Contemporary for a few minutes and went back up to the viewing deck around 8:00. I wasn't paying attention, but 8:30-8:45 is probably when you would want to return to the viewing deck.