"Mysteries of the Castle: Beyond Downton Abbey" is a behind-the-butler look at the Downton Abbey world from inside the real-life English castle that inspired the cult series. (Hazel Thompson/ABC)

Back-to-back repeats of “Glee” (Fox at 8 p.m.) have the glee club kicking off its production of “Grease” with Rachel, Kurt, Santana and Cassandra July at the helm. Then, in a tribute to the 2003 film “Love Actually,” Sam, Brittany, Jake, Puck and Artie all have very different Christmases.

“The Big Bang Theory” (CBS at 8) is also taking advantage of this open prime-time airspace with a repeat episode in which the women’s Vegas vacation is called off and the men invite them to play Dungeons & Dragons, which sparks something special between Amy and Sheldon.

“Wild West Alaska: Grizzly Sized” (Animal Planet at 8) airs an “enhanced” new episode in which a sale at Wild West Guns attracts bargain hunters and a unique cast of characters and, in a dramatic turn of events, Hans’s ex-wife’s horse is threatened by a grizzly bear.

In a special edition of “20/20,” “Mysteries of the Castle: Beyond ‘Downton Abbey’ ” (ABC at 9), the history of the English castle that inspired the BBC cult hit is examined. Hosted by Amy Robach.

On the latest “The Millionaire Matchmaker” (Bravo at 9), Patti is served with two very different clients. One is a globe-trotting entrepreneur and the other is a Florida lawyer, who both seem displeased when a surprise third client is added to the mix.

To make your Christmas shopping look like child’s play, “Buying for Billionaires” (E! at 9) shows us how a few expert personal shoppers seek out uber-expensive gifts for their luxury-loving clients.

Mellie goes into labor on the newest “Gypsy Sisters” (TLC at 9), and Nettie sends Kayla a DVD to plead for forgiveness, while Annie and Sheena help Kayla prepare for a chili cook-off.

FINALE WATCH: In the Season 5 finale of “River Monsters” (Discovery at 9), Jeremy’s quest for the Loch Ness monster takes him to the depths of a Scottish pond and to the volcanoes of Iceland.

“My Big Fat Gypsy Wedding” (TLC at 10) turns to Ireland for more ceremonial shenanigans where not only do we get weddings to gawk over but also christenings and St. Patrick’s Day festivities.

Radio host Howard Stern and comic Adrienne Iapalucci come to the “Late Show With David Letterman” (CBS at 11:35).

Actress Better Midler joins “The Tonight Show With Jay Leno” (NBC at 11:35), with musical guest Michael Bublé.

Actors Carey Mulligan and Scott Foley are on “Jimmy Kimmel Live” (ABC at 11:35), with musical guest Jeff Campbell.

— Rachel Lubitz