Workers at the Pentagon unfurl the American flag at dawn on the 13th anniversary of 9/11. (Reuters)

As flags flew at half-staff Thursday morning to commemorate the 13th anniversary of 9/11, dozens of volunteers wearing bright purple T-shirts were doing jumping jacks at the Inspired Teaching charter school in the District’s Brookland neighborhood. With feel-good hits playing, the volunteers prepared to build a playground for the school to mark the National Day of Service and Remembrance.

In the early evening, hours after the playground had been built, President Obama and first lady Michelle Obama arrived at the school via motorcade to lend a hand.

The couple helped two students fill “playpacks” with books, chalk and other items, which will be given as birthday gifts to children in a nearby homeless shelter, according to a press pool report. They also helped volunteers put a piece of a climbing structure in place, with the president helping to lift the piece and Michelle Obama securing it with a wrench.

The day of service was officially recognized five years ago as a day of doing good deeds in memory of those who died in the 2001 terrorist attacks at the World Trade Center in Manhattan, at the Pentagon in Arlington and on Flight 93, which crashed in a field in southwest Pennsylvania.

The volunteer project at the charter school’s building, which is shared by Lee Montessori charter school, is one of many that took place Thursday throughout the region and the nation. Across the country, hundreds of projects engaged thousands of volunteers who served food to those in need, cleaned shorelines and helped build affordable homes, said Samantha Jo Warfield, a spokeswoman for the federal Corporation for National and Community Service (CNCS).

First grader Dori Wilson and other children bring their homemade ribbon to the Northeast D.C. playground for a ribbon-cutting ceremony. (Sarah L. Voisin/The Washington Post)

In the morning, while President Obama formally honored 9/11 victims and their families at the Pentagon, Vice President Biden took part in a service project at George Washington University assembling care packages for first responders. In Virginia, Gov. Terry McAuliffe took part in the state’s Volunteerism and Service Conference. Online, people shared their acts of kindness on Twitter with the hashtag #911day.

The idea to make Sept. 11 a day of service was conceived in the wake of the national tragedy by New York native David Paine and his friend Jay Winuk, who co-founded the nonprofit group MyGoodDeed to honor those who died by recreating the feeling of togetherness that emerged following the attacks. Winuk’s younger brother was a volunteer firefighter who died trying to rescue others in the World Trade Center.

“My brother Glenn lived his life and died in service to others, so I wanted to get involved in some kind of meaningful tribute to him and all those that were lost in a way that reflected the life he lived,” Winuk said.

Winuk was in New York City on Thursday while Paine stood in the grass behind the District school as nearby volunteers constructed wooden benches with built-in planters. The construction was co-sponsored by the national playground nonprofit group Kaboom, MyGoodDeed and CNCS.

“It’s great it’s not just half-mast flags, but it’s people out contributing; it helps make them and the community and the whole country feel better. It’s healing,” Paine said. “It shows we’re not just victims. We’re standing up for each other.”

After years of lobbying, Paine and Winuk, both public relations specialists, were able to have the day officially designated as part of the 2009 Edward M. Kennedy Serve America Act, named for the late senator who co-authored the bill. The law reauthorized and expanded AmeriCorps and other national service programs administered by the CNCS, which marks its 20-year anniversary Friday.

“[Sept. 11] was a pretty horrific day, but what’s remarkable is the sense of unity that swept the nation, and people really took hold of that,” said Warfield, of CNCS. “When we have this day of service it’s an opportunity to rekindle and continue that legacy of unity and compassion and togetherness.”

The number of volunteers engaged in service projects reached about 30 million participants in 2011, according to a survey by Horizon Consumer Science, funded by MyGoodDeed. The firm projected that more than 40 million people would volunteer this year, Paine said.

To Winuk, the day was “therapeutic,” even when his wife asked if he needed to distance himself from it. “If I had to stand on the sidelines and do nothing in Glenn’s honor, I’d feel a bigger hole in my heart,” Winuk said. Paine said the day offered a chance to do something.

As the Obamas stuffed items into backpacks at the charter school, Obama asked a girl named Madison a question. “What are your favorite subjects?” the president asked the sixth-grader.

“Math and lunch,” she said.

“Lunch is one of my favorite things, too,” Obama said, according to the pool report.

Madison then told Obama that when she heard that an important guest would join the event, she “thought it would be Beyoncé.”

“Then I realized it was gonna be you,” she said. She added that she was happier to meet Obama than Beyoncé.

Kalena Clarke, 37, of Brookland, registered volunteers for Thursday’s playground construction and helped paint. Both of Clarke’s children attend Inspired Teaching, and she said she was thrilled to have a safe and easy place for the kids to play and for community members to socialize.

“I met so many new people who are awesome and great, and today we’re doing the same things and doing them together,” said Clarke, president of the Inspired Families Association.

Head of School Deborah Dantzler Williams said that at the charter school’s old location last school year, the children played in an alley or walked to a park. Williams said she was thrilled her students would have a playground to call their own.

What’s more, she said, they’re learning that “you don’t need a superhero cape to make a difference.”