Except for the baroque chirping from the rafters, the Tabernacle is empty and quiet enough to make one want to whisper. It is 40 minutes before the Island Community Chorus begins to rehearse for its July 6 summer concert. Music director Peter Boak arrives carrying a collapsible stepstool and music stand. He climbs to the stage to arrange and consider.

Gradually and just as stealthily, choir members begin appearing one at a time holding their bottled water and music folders. The singers — a mix of Island teachers, fishermen, business people, artists, retirees, doctors, retailers, lawyers, carpenters — are like long-standing members of a close-knit corps. They take their regular places, sitting on stage composed, waiting, chatting and extending hushed but affectionate greetings when the chairs next to them become occupied.

Some, like Bill McConnell, facilities manager Kenny Romero, Pam Butterick and Sioux Eagle tend to this or that indiscernible task. With consummate efficiency, chorus member Nis Kildegaard, designer of the concert poster, stacks the colorful posters on chairs he has labeled with town names and his printed suggestions of where in each town they can be displayed.

Mr. Kildegaard explains that the chorus has been preparing for this traditional one-time-only performance since April.

“Some people know they won’t be here for this concert, but they still rehearse.” The enthusiasm for this start-of-summer concert is particularly high, he says. “It’s when the summer people can see what we do in winter... It’s kind of a Brigadoon.”

Chairs are moved as the nine-foot-long Steinway grand piano is pushed to the front of the stage. Soon Mr. Boak plays a phrase of Ms. Butterick’s upcoming trio solo for her as she sings. His instruction is offered respectfully, almost conspiratorially.

The sections are now filled with casually dressed, predominantly middle-aged singers. Everyone faces forward, holding the music on their laps or against their chests. Accompanist Garrett Brown, the silent musical genie in the bottle, is now at the keyboard.

It must be seven o’clock because without heralding, the Tabernacle is once again still. Mr. Boak stands on his stepstool in front of the Steinway and everyone rises. After the chorus clenches and releases their shoulders, shakes their arms and huffs and puffs in time, they send aaaahs and oooohs and la la lahs gliding, sliding, up and down the scale.

With a dancer’s grace Mr. Boak moves his arms through the air, delicately conveying specific messages with his fingers — poking the air with an index finger, snatching at air, forming a circle with his thumb and index finger. The chorus understands it all and responds.

Since its founding in 1996, the non-profit Island Community Chorus, about 120 strong, has advanced from its first seasonal performance of Handel’s Messiah to year-round programs that have included the Requiems of Gilbert Faure and John Rutter, a full Bach cantata, works by Benjamin Britten, Daniel Pinkham, and Felix Mendelssohn’s oratorio, Elijah, with orchestra. These are all part-time, amateur singers elevated by their director to concert-worthy performances. Still, membership requires no audition, just dedicated and regular attendance at its Monday night rehearsals held year-round except for a two-month hiatus.

“Sopranos, page nine.”

Mr. Boak voice is relaxed, conversational and he seems more an advocate than a maestro. “Bottom line. One, two, three, four and one.”

The sounds of Wana Baraka, a joyful Kenyan folk song, resound. But it is phrase-by-phrase, section-by-section, start and stop. Directions are gently given.

“Make sure you have that accent. Are you aware that this is a dialogue? If you don’t make it happen the same every time, it gets sloppy. Okay, take it from the top, two, three, and . . . .”

There is no conversation between Mr. Boak and his longtime friend and accompanist Mr. Brown. None are necessary. Mr. Brown seems to anticipate what’s needed, stressing a pitch or tempo.

Mr. Boak taps his sandaled foot, sings along and moves into the sections, leaning in as he says, “Could I just hear . . . ?” He signals a stop. “Don’t read the music so intently. That can’t be fun for you. That’s absolutely unnecessary. Trust yourself.”

And then they all sing out, this jeans and shorts, T-shirted, sandal-footed hodgepodge group of spectacular amateurs, making a sound so jubilant as to send one twirling around the empty aisles of the Tabernacle in celebration.

The Island Community Chorus Summer Concert is Saturday, July 6, beginning at 8 p.m. at the Tabernacle in the Oak Bluffs Campgrounds. Free will offering. One show only.