In celebration of the milestone 20th season, "American Idol" greats Ruben Studdard, Scotty McCreery, Jordin Sparks, David Cook, Lauren Alaina, Kris Allen, Maddie Poppe, Laine Hardy, Willie Spence, Caleb Lee Hutchinson, Grace Kinstler, Laci Kaye Booth and additional surprise guests will reunite on the Idol stage for a night of unforgettable performances.