The Zimbickis camped under a blue plastic tent. Two American flags attached to one side of the tent blew in the wind.
Some people attended for the first time.
Jim and Rose Gardner from Henderson, Md., and Joe and Barbara Little from Denton, Md., said they'd never celebrated Independence Day on a battlefield.
They sat under a shade tree with a view of the lone New York State Monument on a hill.
"It's moving standing on the same ground as a battle took place," said Jim Gardner, 58.
Barbara Little, 48, said she visited Antietam in September and was impressed. One day, she saw an advertisement for the MSO concert.
"I said we've got to come back for this," she said. "And we couldn't have asked for a prettier day."
Some people got caught up in the action by accident.
Phil and Zina Atrakchi drove from Rockville, Md. ,to focus on the battlefield and found out about the festivities after they arrived.
They took photos each other in front of a row of four cannons.
"It's a feeling of being in a place where a lot of people died to make the country the way it is today," said Zina Atrakchi, 25. "It is what the ancestors left for the new generation, which is a great accomplishment."
Some people came to have fun in the sun, like Stan Hott, 22, of Hagerstown, Ken Kipe, 20, of Chambersburg, Pa., and Sally Markel, 18, of Williamsport, who sat and ate on colorful blankets while waiting for 20 more friends to arrive.
"It's cool," said Hott, who attended 10 concerts there. "I like it. What better place to do it?"
One thing everyone seemed to have in common was that they came out to relax.
Bob and Deb DeMine, of Williamsport, arrived at 2:30 p.m.
"We did not put blankets out earlier, but a lot of people we know did," said Deb DeMine, 35.
They sat in the shade across the street from Dunker Church.
"We have food for the whole evening and games, chairs, blankets and pillows," she said. "We forgot our flashlight this year."
They brought their children, Alex, 6, Kevin, 8, and Megan, 3, and Deb DeMine's mother, Beth Megill, 65, of Frederick, Md.
"It is so much better than going to D.C.," Deb DeMine said. "(In DC) the traffic is a hassle. Here you can get close enough to the music without getting here at 6 a.m. And it has trees. D.C. doesn't have trees on the Mall."