"Belles de Jour" is the 66th episode of the CW television series, Gossip Girl, as well as the season premiere of the show's fourth season. The episode was written by Joshua Safran and Stephanie Savage and directed by Mark Piznarski. It originally aired on Monday, September 13, 2010 on the CW. The episode filmed several scenes in Paris, a move that was heavily promoted by the show's network and commended by critics. A promotional video entitled "Oh Mon Dieu" used a French rendition of Nancy Sinatra's "These Boots Were Made For Walking" to advertise the season premiere.

The episode received generally favorable reviews, with critics and fans praising Katie Cassidy for her performances as Juliet Sharp. The fourth season premiere opens with Serena van der Woodsen (Blake Lively) and Blair Waldorf (Leighton Meester) spending their vacation in Paris to move on from their troubles. Vanessa Abrams (Jessica Szohr) returns to New York and finds Dan Humphrey (Penn Badgley) living with Georgina Sparks (Michelle Trachtenburg) and a baby, while Nate Archibald (Chace Crawford) attempts to move on from his breakup with Serena, he befriends Juliet Sharp (Katie Cassidy), a girl with ulterior motives for helping him.