“Treehouse of Horror XX” (8 p.m., Fox): It has been a Halloween tradition for 20 years

“Treehouse of Horror XX” (8 p.m., Fox): It has been a Halloween tradition for 20 years and, for once, it actually airs before Halloween. “The Simpsons” present another set of frightening — and funny — tales, including one that has news anchor Kent Brockman transforming into an enraged zombie after devouring one of Krusty’s fast-food sandwiches. Other stories see Lisa channeling Alfred Hitchcock and tavern-owner Moe serving up drinks flavored with Homer’s blood. Mmmm, blood beer.

“Mad Men” (10 p.m., AMC): The marvelous third season of “Mad Men” continues as Peggy and Paul compete for an account. Our money is on Peggy.