There's a concert I'd like to go to in September, help me find a way to bring it up to my mom?

I would first talk to your friend about the hotel room and figure out how you are going to handle that. Then sit down with her and ask her if you can go if you pay for it all. Do it in a mature, rational way even ask her to think about it. That way nothing has to be decided at that moment. And if she says no, try to accept it that way she has more incentive next time to say yes after seeing you handle the situation maturely.