Who Books Shows?

Does anyone know who takes the first step in booking a show is it the venue that hooks up with the promoter or vice versa? The post said not all us date are out yet so I was wondering who books the show. I know some people here at the arena in buffalo so if the venue is contacts the promoter that would be cool its a long shot but worth the try. I just cant stand missing my #1 and 2 favorite bands together lol! I wish my mom would just let me go up to toronto but it is a school nite lo!