Every Gossip Girl episode title is a word play on the title of a movie. This episode's refers to Belle de Jour (1967).
Was filmed in Paris.
The original idea was that Blair (Leighton Meester) would push Serena (Blake Lively) into the Seine, but the producers realized that would seem too hostile, and fortunately opted for the fountain instead.
Clémence Poésy (Eva Coupeau) ends up playing the wife of Tom Hiddleston in The Essex Serpent (2022), playing Stella Ransome and Will Ransome respectively. Sebastian Stan (Carter Baizen), who appears in several earlier episodes of the show, has worked with Tom in the Marvel Cinematic Universe, a group of movies based on the Marvel Comic Books. They play Bucky Barnes/Winter Soldier and Loki respectively.
Katie Cassidy (Juliet Sharp) and Willa Holland (Agnes Andrews, who doesn't appear in this episode) end up working in the same TV shows, set in The Arrowverse, Arrow (2012) and The Flash (2014), playing Laurel Lance/Siren X/Black Siren and Thea Queen/Speedy respectively.