The Simpsons' 18th season aired from September 10, 2006 to May 20, 2007. The season contained seven hold-over episodes from the season 17 production line. Al Jean served as the Showrunner, a position he has held since the thirteenth season. The season finale, "You Kent Always Say What You Want", was the series' 400th episode. Additionally, the Simpsons franchise celebrated its 20th anniversary, as it has been on the air since April 1987, beginning with shorts on The Tracey Ullman Show. Season 18 included guest appearances by Metallica, Tom Wolfe, Gore Vidal, Michael Chabon, Jonathan Franzen, Fran Drescher, The White Stripes, Kiefer Sutherland, Mary Lynn Rajskub, Richard Lewis, Phil McGraw, Elvis Stojko, Natalie Portman, Jon Lovitz, Betty White, Eric Idle, Sir Mix-a-Lot, Stephen Sondheim, Cristiano Ronaldo, Meg Ryan, Andy Dick, Peter Bogdanovich, James Patterson and others.