The way it works it, you can camp in the major camping areas, such as Camp C (camp which allows cars and the such) on any day.. were some people there last year who'd been camping for a week (and I arrived on tuesday) - problem with this campsite is it gets full very fast, and is pretty much very dirty and skanky by the tuesday / wednesday of the festival week.
The campsite for non-car owners opens on the Wednesday - security patrolled the tuesday (even in pouring rain) so we couldn't camp there until wednesday at 5am - however, this campsite was the better of the two for me as it's fresh and not pounded by car stereo's.
Not sure how it'll work next year with the stage changes and such though, concerning the wednesday non-car campsite that is.