John Eaton is Washington’s foremost pianist, vocalist, musicologist and humorist rolled up into one extraordinary performer. Concluding his Barns of Wolf Trap journeys this season through “The Roaring Twenties” and “The Swinging Thirties,” he pays tribute to “The Fabulous Forties” and the composers who kept the nation singing and dancing during and after World War II.

“The theme I’ve developed is that you can measure the evolution of jazz by the way mainstream writers wrote,” he says. “You had Thelonius Monk and Charlie Parker coming up during the ’40s, so it’s a period when music looked backward and forward at the same time.