I think it was the one where Ray spys on Debra through the window and catches her crying. She admits she likes to cry and tells Ray that one of the things she does to get her in the mood is to listen to the song from Ice Castles. Later, to see if it'll work for him, Ray puts on the Ice Castles song and sits on the couch with a box of tissues next to him. He makes a "crying face" but to no avail, so he tunes in to Voulez Vous and starts dancing.
Oops. I thought you were asking, "When did Ray DO that dance...." Sorry!