Many of those who read names at the memorial were children, either born after the attacks or too young to remember the friends and family members who died. Ms. Reina, whose husband worked at Cantor Fitzgerald, the bond trading firm that lost 658 employees in the attack, was pregnant on 9/11. Her son is part of an entire generation that has been born in the shadow of that day and has only received its legacy secondhand.

Ariana and Briana Mendoza, 13, came to Lower Manhattan from the Bronx with their sister, Dephaney, to pay their respects. “I was only 2 when it happened,” Dephaney, 22, said. “But I have learned a lot about it, and now I am teaching them.”

Nearby, Luis Gonzalez, 41, of Staten Island, stood staring up at One World Trade Center, the tower built upon the ruins of ground zero. He carried a poster of the old twin towers that now hangs in his bedroom. “I come out of respect,” he said.

Many others went to smaller but no less emotional events across the country for similar reasons. At the stair-climbing event in Chicago, Marisa Price, a firefighter from Westmont, Ill., said she felt a deep connection to the emergency workers who gave their lives during rescue efforts.

“A lot of them were my age at the time and made the ultimate sacrifice,” Ms. Price, 25, said. “It’s something we’re all willing to do but don’t want to do.”

At one of the day’s marquee college football games, the Ohio State University marching band performed a patriotic tribute during halftime, moving through formations that included an American flag, the Statue of Liberty and a bald eagle.

Memorials were also held across the globe. Outside Buckingham Palace in London, during the changing of the guard, the guardsmen’s band played “The Star-Spangled Banner,” as it had after Sept. 11, 2001. At the NATO headquarters in Brussels, the secretary-general stood in front of a piece of twisted metal from the World Trade Center for a moment of silence.