If you want great festivals that last more than a weekend then consider Mount Airy and Clifftop. People start arriving at Mount Airy (North Carolina) about the tuesday before the official weekend, and the standard of musicianship that builds up over the next few days is astonishing. Clifftop (West Virginia)lasts even longer, the sessions and jamming start the previous weekend, and if you arrive then you get your pick of the best camping spots. The Old Time stringband content of both these festivals is great, with many fine musicians taking part in the competitions over the official weekend.
I'll be at Clifftop this year - it's been far too long since my last visit (2002), and I'm really looking forward to jamming with people I met at other events - I know they'll be there!
Ray