This may not have purely been a target of opportunity. It was the final performance of the festival, but I have to think it's likely the Saturday night crowd would have been bigger than the Sunday night crowd. That leads me to the date as having some significance. Certainly 10/1 has significance, as 9/11 can be permutated to 10/1 by adding the first two digits.
Since the alleged shooter checked in on the 28th, and housekeeping reported no weapons in the room, all of the weapons would have to be placed into the room the day of the shooting. Considering there were 10 weapons, I'm not sure how this would have been accomplished. Someone would have to have stayed in the room the entire day.
My initial inclination is that the fact nobody has mentioned the date as having significance is a tell, and we may not be getting much of the story.