Originally posted by Partagas: The Headline for the Financial Times headline dated Novemeber 15,1979 reads: "Building an Empire: Corleone Proposal for New Immobiliare". The Opera was after the Immobiliare Mtg -- Eastertime. Thus, Mary, et al killings tookm place in Spring of 1980.
Your deductive reasoning is right, but look carefully at the poster for the opening of the opera. It states the current season as "1978-1979".
