More details and the full schedule is at http://www.dominionriverrock.com/schedule/
The Riverock festival is free and includes a bouldering competition (there may be an entrance fee to compete). But the concerts are definitely free.
I've never been, but I always wanted to go. I'm planning to drive down Saturday morning and have room for 2-3 people in my car. If people want to stay overnight, I booked a room at the Crowne Plaza Downtown, less than a half-mile from the park for Saturday night. Otherwise I'll just go for the day on Saturday.