How do you celebrate a music director’s two decades leading an orchestra? If you’re the Michigan Philharmonic, you do it with a series of “Happy 20th NANiversary” concerts for Nan Washburn. The Plymouth-based orchestra opened the season in October with some of Washburn’s favorite music: Berlioz’s Roman Carnival Overture; Hindemith’s Symphonic Metamorphosis on Themes by Carl Maria von Weber; Wael Binali’s Earth: Plunder, Wound, Renewal, Hope, written for the U.N. Climate Change Conference in 2012; and Barber’s Violin Concerto, Op. 14, with soloist Danielle Belen. The season concluded in April with “PhilPalooza2,” a bursting-at-the-seams concert featuring the orchestra performing side by side with the Michigan Philharmonic Youth Orchestra and guest performers including the Plymouth Community Band, saxophonist Zach Sheman, and composer and bassist Rick Robinson. “PhilPalooza2” took place at a gym in the Plymouth Arts and Recreation Complex—the only space big enough to fit all 150 musicians plus audience.