Mysteries of the Castle: Beyond Downton Abbey (9pm, ABC) - A special edition of 20/20 looks at the history of Highclere Castle in Newbury, England, the castle that inspired the PBS Masterpiece series Downton Abbey.
The Secret Life of Walter Mitty (8pm, Turner Classic Movies) - A 1947 adaptation of James Thurbers short story about a milquetoast proofreader ( Danny Kaye) who finds refuge in daydreams of bravery and heroic flights of fancy, including a Wild West gunslinger, a riverboat gambler and an RAF pilot. The 2013 remake starring Ben Stiller opened in theaters this week.
River Monsters (9pm, Discovery) - In the Season 5 finale, Jeremys search for the Loch Ness monster takes him to the depths of the Scottish lake and the volcanoes of Iceland.
The Women of SNL (9pm, NBC) - A repeat of a 2010 special spotlighting the female performers of Saturday Night Live, including memorable past sketches featuring Rachel Dratch, Tina Fey, Julia Louis-Dreyfus, Cheri Oteri, Maya Rudolph, Amy Poehler, Molly Shannon, Kristen Wiig and others.