With a silky afternoon mist falling on Fort Dupont Park in Southeast Washington, the band takes the Summer Theatre stage for sound check. The procedural thump of the kick drum seems to flush dragonflies out from the neighboring tree line, oaks and maples.

Up the hill, different sounds. Ice sloshing around Igloo coolers. Two-liter bottles of Pepsi hissing open. Plastic forks rattling out of their boxes. It’s 3 p.m. and a dozen or so have arrived early enough to claim their spots on the sloping lawn. Each summer, they spend six consecutive Saturdays at Fort Dupont picnicking, partying, cutting up and getting down. With casual pride, they call themselves “the Hillside Crew.”

This summer marks 40 years of free concerts at Fort Dupont Park. Carlos Evans remembers 38 of them. “The trees you see down there used to be about this tall,” he says, tapping the brim of his periwinkle ballcap. He’s seen too many acts from the top of this hill to count. Gil Scott-Heron. Mother’s Finest. Bobbi Humphrey. Stacy Lattisaw. The Ohio Players. Zapp. Jill Scott.

Too many people to count, too. These days, Fort Dupont’s biggest concerts quietly draw a whopping 11,000 fans, and everybody seems to know everybody else. Leave your blanket. Nobody’s gonna mess with your stuff. We’ll keep an eye on the kids. Need more ice? Want some fruit salad? Wanna dance? The closest thing to violence anyone in the Hillside Crew has ever seen at Fort Dupont was a man shaking his groove thing so hard, he slipped and went somersaulting down the slope in his white linen slacks.

“The party to remember.” That’s how jazz singer Roy Ayers put it in “Ft. Dupont Park,” a song he wrote about this place in 2004. “I have had some great experiences there,” says Ayers, who will make what he calculates to be his 10th Fort Dupont appearance this weekend. “It’s a great atmosphere for the summertime where people can go and just let out their frustrations.”

Four hours before showtime, Evans rolls down the windows of his SUV and pumps Ayers’s anthem from the speakers. Its syrup-sticky melodies carry over to Kenneth and Raydean Thomas of Fort Washington who have pitched a half tent on the hill’s summit.

The couple have been coming to Fort Dupont for a decade, lounging in their lawn chairs until the sun sinks down and the music fires up. Then it’s time to two-step in the grass. Like so many Fort Dupont faithful, they first learned of these concerts by word of mouth.

“D.C.’s biggest, best-kept secret,” Raydean says. “That’s what it is.”

Four decades ago, many of Washington’s biggest parks doubled as dance floors thanks to “Summer in the Parks,” a program implemented by the National Park Service in 1968 following the assassination of Martin Luther King Jr. and the subsequent riots that destroyed so many District neighborhoods.

In order to draw young people out of the streets and into the parks, free concerts were held in Meridian Hill Park, Anacostia Park, on the Mall and at other locations. Today, only the annual summer concert series at Fort Dupont, Fort Reno in Northwest and Carter Barron in Rock Creek Park have survived through the decades.

The music began at Fort Dupont in 1972 when the weekend concerts held at Anacostia Park were relocated deeper into the heart of Ward 7. Intended for families that couldn’t afford trips to the Kennedy Center, Fort Dupont hosted jazz, blues, soul, funk, go-go, gospel and more, every Friday and Saturday night, Memorial Day to Labor Day.

As the ’70s slid into the ’80s, Fort Dupont focused on jazz, bringing Ahmad Jamal, Art Blakey, Nat Adderley, Ramsey Lewis, Sun Ra and other jazz giants to the stage. When Carter Barron began charging admission for some of its performances that decade, Fort Dupont remained free, drawing as many as 25,000 fans per night.

But those numbers — concerts and concertgoers — thinned over time. Today, Fort Dupont manages to host six performances each summer on a $205,000 budget that stretches wide enough to cover the acts, the sound, the lighting, overtime pay for park staff and assistance from the U.S. Park Police.

Park manager Julie Kutruff and ranger Kevin Barry are overseeing the concert series for their fifth summer and have made each penny count. On Saturday mornings, they divvy up the seven-acre swath of grass in front of the Summer Theatre stage with an arsenal of 32-gallon Rubbermaid trash cans and spools of yellow caution tape, creating makeshift aisles to keep the throngs organized. They also put a big yellow donation box out near the stage.

One expense they don’t have to worry about is promotion. “We have this legacy audience that comes whether we advertise or not,” says Kutruff, whose annual publicity push amounts to an e-mail blast and 1,500 fliers distributed in the neighborhoods surrounding Fort Dupont. They try to fill the park every Saturday, while carefully trying not to overfill it.

And that wasn’t always the case. Tina Fortunato was hired to book the series 11 summers back when attendance was slipping. The Annapolis-based concert promoter noticed old-school funk and soul acts getting the biggest cheers from Fort Dupont’s loyal, mostly middle-age, almost entirely black audience. So she booked more of them. The Stylistics. The Delfonics. S.O.S. Band. Lakeside. “All of a sudden, we were packed,” Fortunato says. “Absolutely packed.”

That formula still works. A local act always opens, a legacy act usually headlines and for the third consecutive summer, the season kicks off with a performance from members of the Boys & Girls Clubs of Greater Washington. (The Hillside Crew raves about this year’s youth concert on July 14.)

Fortunato says that despite the budgetary constraints, she has artists knocking down her door, eager to perform in the open air for appreciative fans.

“All the people you see out there, they’re all having a good time,” says Herb Feemster of Washington’s own Peaches and Herb, who will perform at Fort Dupont for the first time time Aug. 11. “You say, ‘Well, damn! I wanna be up there.’ ”

The same goes for Heatwave, the disco greats who made their first-ever Fort Dupont appearance on a recent Saturday night. “You see the appreciation, you feel it,” says Heatwave singer Keith Wilder before the band’s July 21 show. “It just drives you to kick butt as if you’re getting paid 150 dollars a ticket. It does!”

The rain eases up around 7 p.m., and fans finally start streaming into the park. Tank tops. Tube tops. Redskins jerseys. Feathered fedoras. Curly W’s. Kangols turned forward, backward and sideways. Chuck Brown memorial T-shirts, faded, as if they’d been worn ever since the Godfather died.

Barry estimates about 800 in all. Only one-tenth of the average crowd size, but not bad, considering the weather. The rangers stand sentinel in the crowd, their iconic straw hats protected by translucent shower-cap-ish covers. They think they look “dorky,” but the hats cost $80 a pop — too high a price for looking cool in the rain.

A local quintet, Nu Era, takes the stage first. They’re an everlasting Gobstopper of a group, crooning old-school and older-school R&B covers from New Edition and the Temptations. When they dash off stage at 8:45, the mist thickens into drizzle. Funkadelic’s “One Nation Under a Groove” oozes from the speakers, and umbrellas become dance partners. Scores of Gene Kellys are singing George Clinton in the rain.

At the stroke of 9, Heatwave appear in sparkly suits, doing high-step dance routines as if repeatedly stepping in wads of hot sidewalk gum.

“Reminds me of the days when the Redskins played in the mud and beat everybody,” says Wilder between songs, peering out to the mushy field. When his band bounds into “Boogie Nights,” a few glowing orange dots sway from side to side on the tree line, cigarette smokers puffing in the dry shadows.

The night sky seems bigger up on the hill, orange and purple at the same time, like some impossible bruise. Down below, the stage pulses with colored lights like a box of jewels tipped on its side. For the Hillside Crew, the music is loud enough for dancing, but quiet enough for chit-chat.

Donna Green asks her friend Tina Henson, “Where’s Ms. Giant?” They don’t know Ms. Giant’s real name — she’s a grocery store clerk who usually drops her blanket off in the morning, leaves to work her afternoon shift and then swoops back into the park around 8 with her name tag still pinned to her shirt. Rain must have scared her off, they say. Her loss, they say. Isn’t raining anymore, they say.

“And ain’t no mosquitoes tonight, either,” Green says.

“They don’t like your dancing,” says Evans.

She slugs him in the arm. He laughs. They both bounce to “Groove Line.” Nobody falls down the hill.

The summer concert season at Fort Dupont Park is underway. Below are the remaining shows:

July 28: Roy Ayers; Secret Society

Aug. 4: “In Gratitude: A Tribute to Earth, Wind & Fire” featuring the Graingers, WaWa LaGrande, Gene McBride, Eddie Baccus and Greg Boyer; Mousey Thompson and the James Brown Experience; Jeffrey Walker.

Aug. 11: Al Johnson and His A-Team; Peaches and Herb; Skip Mahony and the Casuals; HALO.

Aug. 18: Klymaxx featuring Bernadette Cooper; Be’la Dona

Fort Dupont Park is at 3600 F St. SE. Concerts begin Saturday evenings at 7:30 or 8 p.m. depending on the date. Admission is free.