The way Graspop works is they have four stages: Main stage, Marquee 1, Marquee 2 (Around half the size of Marquee 1), and the Metal Dome.
Metal Dome and Main stage run at the same time, but Metal Dome only has about five bands a day, and ends around maybe 3/4 o clock. The Marquess run all day and are also at the same time, and whenever a band finishes on Mainstage os the MEtal Dome, the bands in the Marquees start. With the billing the way it is on the poster, I am willing to bet Volbeat will headline a Marquee while Korn play main support on the Main Stage. Same with Cavalera Conspiracy over Avenged. The Main Stage bands are slightly lower on the bill, but have the best crowd to play for. Thats the way the line up has been the last two years.
Yesterday don't mean SHIT.