“Our Town,” Thornton Wilder’s three-act play set in a small town in New Hampshire a century ago, turns 75 Tuesday. It was Jan. 22, 1938, when the main character, the Stage Manager, first guided an audience through the play — staged at the McCarter Theatre in Princeton, N.J. “Our Town” has had a great run ever since. It won Wilder the second of his three Pulitzers, and remains one of America’s most-performed plays. (Theater lore has it that every night it is staged somewhere around the country.) “Our Town” takes place over three days in Grover’s Corners: May 7, 1901, an ordinary day in the town; July 7, 1904, George’s and Emily’s wedding day; and the day in the summer of 1913 when Emily is buried. (From her grave, Emily makes a brief and disappointing visit back home for her 12th birthday, Feb. 11, 1899.) The Stage Manager informs us that the play will tell generations a thousand years hence: “This is the way we were: In our growing up and in our marrying and in our living and in our dying.”