Saturday: After the indie/alt-rock-heavy SoundTown festival went bust, Somerset Amphitheater is going back to the metal camp-out formula that was long a mainstay at its riverside grounds. Iowa's masked avengers Slipknot, who played there during the Ozzfests and 93X Fests of old, chose the Wisconsin site as one of two spots to stage its inaugural one-day Knotfest. This will be the band's first area show since the 2010 death of co-founding bassist Paul Gray. It will also be the first shows for Virginia-bred death-metal favorites Lamb of God since singer Randy Blythe was freed last week from a Prague prison after being detained for a month in a 2010 incident in which a fan was thrown off the stage. The rest of the lineup includes the Deftones, Machine Head, System of a Down singer Serj Tankian, Cannibal Corpse, Prong and more. (2 p.m. Sat. $45. Somerset Amphitheater, Hwy. 64, Somerset, Wis. Also Fri. in Council Bluffs, Iowa.