But for the intervention of the censors, Giuseppe Verdi’s Un Ballo in Maschera (“A Masked Ball”) would have entered the repertory as an opera set in 18th-century Sweden. The trouble was that it culminated in a nonnegotiable no-no: the onstage murder of a king. And so, the action migrated to colonial Boston, governed by a British earl who could be dispatched without compunction. The change is immaterial to the drama, but the Mozartian grace notes of the score only make sense in the original context, which has been gaining ever greater favor in recent decades. It’s inevitably the Stockholm option that will be exercised in the concert performance at the conclusion of the Birgit Nilsson Days 2024, held on the grounds of the farm where the future prima donna assoluta grew up working the land and the livestock. Intensive master classes are the chief business of this annual event, but for the finale, the watchword is excitement. Michael Fabiano and Joyce El-Khoury, who delivered the goods last summer as the lovers in a concert performance of Puccini’s Tosca, are now on point as the king and the woman he loves, who just so happens to be his best friend’s wife, too pure of heart to betray her husband but not too pure to be tempted. Ah, opera! —Matthew Gurewitsch