When you are going to a large concert which clothes should you wear?

I'm going to a oneday festival with bands like Gojira, Millencolin, Protest the Hero and Avenged Sevenfold. I HAVE NO IDEA what to wear, and i don't have any band t-shirts either. Its going to be 12-15 celsius and some rain.. Ppl who attend a lot of concerts, HELP ME ! :D