Don Carlos was an opera composed by Giuseppe Verdi in the 19th century.
In 2375, The Doctor wanted to rehearse a duet from Don Carlos on the holodeck, but it was still occupied by Tom Paris running his Captain Proton holoprogram. (VOY: "Night")
The Doctor performed a duet from the opera, "Dio, che nell'alma infondere", aboard the USS Voyager for an audience of Voyager crewmembers and a number of fascinated Qomar in 2376. He later gave small holograms of him singing the same song to Qomar admirers, even singing a duet with his small holographic self. (VOY: "Virtuoso")
According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 215), this opera was completed in 1884.