A familiar face returning to American Idol! Keith Urban is returning as a guest mentor for an upcoming episode of the singing competition. Urban will serve as a guest mentor for the season 21 finale on May 21st. “Headed back to @americanidol next week !!!!!” Keith posted on his Instagram Stories. American Idol's three-hour season 21 finale airs on Sunday, May 21, at 8 p.m. ET on ABC. Who has been the best American Idol mentor in the show’s history?