This is Trieste's main opera house, built under Austrian rule in 1801. It is of interest not only for music lovers, but also for its architecture—the interior was designed by the architect of Venice's La Fenice, Gian Antonio Selva, and the facade was designed by the architect of Milan's La Scala, Matteo Pertsch. You'll have to go to a performance to see the interior, since guided tours are not given to individuals.
The opera season here runs from October through
May, with a brief operetta festival in July and August.