Season 9: Episode 202: The One with Christmas in Tulsa
Chandler has to spend Christmas in Tulsa to get paperwork done, and realizes he's the only one in his group of Friends who hates his job. He remembers past Christmases in a series of flashbacks from earlier Christmas episodes, and decides to send his subordinates home. One (Wendy) stays behind. Monica is not comfortable with Chandler working alone with her. He believes she has nothing to worry about, until Wendy makes a pass at him. He turns her down, professing his love for Monica. She asks why he's not with his wife on Christmas, which prompts a series of flashbacks about his relationship with Monica; also prompting him to quit his job and return to New York.