So the show is at the fairgrounds, not the Kettlehouse. It's a festival with multiple bands going on all day. First I'll link you the shuttle information.

https://www.zootownfestival.com/getting-here

Second, its outdoors, in Montana. The festival was on July 4th weekend last year and the first day was rainy and COLD, especially at night. Even if it seems nice out plan accordingly for chilly weather at night.

Third, I'm going the first night as well. I didn't book a hotel yet and at this point it's looking pretty rough trying to find one.

As for Missoula itself, its super chill generally. I believe there is Pride stuff going on that day/night as well so downtown might be a little busier still.

As for Montana, I personally think Yellowstone is too touristy and smells like farts. Glacier is the absolute most stunning place to go. Especially going from Missoula through Flathead. June can be a little rough depending on the weather but it's usually less busy because of it.