The Met: Live in HD will return to Guild Hall on Saturday at 1 p.m. with Mozart’s “Cosi Fan Tutte.” A two-act opera buffa, or comic opera, “Cosi Fan Tutte” is set in Naples in the 18th century. Don Alfonso wagers two young soldiers that their sweethearts will prove unfaithful if given the chance. The men accept the bet and tell their sweethearts, who are sisters, that they have been called to the front and must leave Naples. Switched identities and comic confusion follow.