ABC has announced that the season finale of Twin Peaks will air at 10 p.m. on Wednesday, May 23, preempting Equal Justice. ABC is moving the season-ending episode of the new serial thriller from its usual Thursday slot back to Wednesday in order to include its potentially large audience in the May ratings sweeps, which conclude May 23. . . . On Monday, cable channel Nickelodeon will kick off a “The Seven Day Donna-thon,” during which 140 episodes of The Donna Reed Show will air back-to-back nightly from 8 p.m. to 6 a.m., culminating on Mother’s Day, May 13, with an instructional special, “How to Be Donna Reed” . . . . Candice Bergen, star of CBS’s Murphy Brown, will host the May 19 season finale of NBC’s Saturday Night Live. It will be her fifth appearance on the show, including two appearances during SNL’s first season of 1975-76.