This year, approximately 90 historic sites in 21 states were expected to participate in Park Day.
The project was underwritten with a grant from The History Channel. It also was recognized by the U.S. Department of the Interior as a "Take Pride in America" event.
At Antietam National Battlefield, volunteers began arriving well before the 9 a.m. start time.
"The response from the public has been great," said Debbie Cohen, park ranger at Antietam. "I'm guessing about 100 people will show up today. And it's a good mix - from Preservation Trust members, to students who need community service hours, to people who just want to lend a hand."
The volunteers came ready to get dirty, wearing old clothes and heavy boots. Led by park staff, they worked in groups to remove trash, build and repair fences, and plant trees.
Cohen said that the volunteers now have a vested interest in Antietam.
"Today, they'll make a strong connection to a resource, and that's something that doesn't happen if you're just passing through the park," she said.
"A year from now, they'll come back to see how much the tree has grown that they planted today. They'll bring their grandchildren and show them the fence they mended. They'll feel they're a part of Antietam."
Among those volunteering for Park Day was Lori Floro of Hagerstown.
Floro is employed with the National Park Service, but said she seldom has an opportunity to get out to see the parks.
"So I thought this would be a good way to come out, enjoy the day and lend a hand," she said.
Caleb Cheshire, 11, of Morgan County, W.Va., was hoping his volunteer work would earn him his environmental science badge as a member of the Royal Rangers.
"It was tough getting out of bed on a Saturday," he said. "But it's worth it."
His mother, Crystal Cheshire, said Caleb is starting to learn about the Civil War at school, "so, in addition to the volunteerism, I thought this might be a bit of an education. It's also something we can do together as a family."
Also participating in Park Day was Harpers Ferry (W.Va.) National Historical Park.
According to David Fox, park ranger and volunteer coordinator, Saturday's work included opening a path for a new trail and new outdoor exhibit panels.
"To do this, the volunteers will help us cut down brush, trees, old fences and fence posts," Fox said. "All of this material will be either hauled away or turned into wood chips on site. We will have park service chain saw crews on site to take down the big trees."
The work was being done on land that the public never has had access to, Fox said.
"This is land that was crisscrossed by many soldiers during the Civil War," he said. "The most well-known soldier to walk this ground was Stonewall Jackson. In fact, the site where we will be working today was the center of Jackson's line when he forced the largest surrender of Union troops during the entire Civil War - approximately 12,500 men."
Fox estimated that between 15 and 25 volunteers would participate in Park Day, as well as 10 park staff members working in support of the volunteers.
"This work by these volunteers means that visitors will be able to enjoy this green space and learn about this Civil War history forever," he said. "And for the first time, visitors to the area will be able to view the military tactics from the Confederate perspective."