The one-hour 18th season finale of "The Simpsons" (which also marks the show's 400th episode) features a rap about dental hygeiene from Luda-Crest (voiced by rapper Ludacris) and some good times with Krusty the Clown. "The Simpsons" season finale airs Sunday, May 20 at 9 p.m. ET on Fox. (Fox)