A Night at the Opera was a film starring the Marx Brothers from the early 20th century.
This film was shown during a movie night on the Enterprise in 2153. Sim asked T'Pol to accompany him there, but she refused. (ENT: "Similitude")
Peter Allan Fields, the writer of DS9: "The Circle", based the scene in Kira's quarters, in which everyone comes to say goodbye to her, on a scene in A Night at the Opera. (Star Trek: Deep Space Nine Companion)