Is it indoors or out? Is it a fundraiser for anything in particular? What type of music do you expect to be performed for the most part?
With the exception of recitals and classical performances, concerts are usually casual. Remember casual doesn't mean sloppy though. Nice jeans or a funky skirt, a top that will allow you stay cool if your dancing, and comfy shoes or boots. You'll be on your feet alot). I would go with a more funky style than conservative.