One who is committed to regularly attending music festivals. Commonly used for people who make attending music festivals into an event by camping out with friends, particularly during summer.
It does have the connotation of someone who gets wasted and parties while attending these festivals, but the overall meaning is based on simply frequenting festivals.
"Hey, dude, we're all going down to Austin for Austin City Limits music fest. Wanna come?"
"No, that's okay. I'm not much of a festival-head."