Even if you can't be there for an opera (opening with a gala to end all galas every sacrosect December 7), attend a concert or dance performance there or at least visit the little museum (left of the theater), if only for a chance to see a strand of Mozart's hair, Toscanini's baton, or a peak into the sumptuous Neoclassical auditorium with its six tiers of balconies and galleries.
It is worth visiting any form of performance there.