The Hallmark Channel is offering a 12-hour Mary Tyler Moore Show marathon Saturday. While you won't be seeing the "Chuckles Bites the Dust," you can several of the show's other memorable episodes, and catch glimpses of notable guest stars — including Jerry Van Dyke (1 p.m. ET), Henry Winkler (2:30 p.m. ET), Bruce Boxleitner (4 p.m. ET) and Penny Marshall (9:30 p.m. ET) — before they were notable. The biggest highlight, though, may be the 8 p.m. ET episode, which features a guest appearance by newsman Walter Cronkite.