LOS ANGELES — Samantha Denaro knows what happens when she tells an adult her birthday. There is a pause, sometimes a gasp. Then, more often than not, an apology.

“I’m so used to it,” Denaro said. “At this point, that’s probably the No. 1 reaction I get. I don’t ever know what to say.”

Usually, she just tells them it’s fine.

Denaro was born Sept. 11, 2001, one of 13,238 people born in the United States that day. On Friday, they turned 25 on a date most Americans have never needed explained, one that still carries the weight of what ended that morning, when nearly 3,000 people were killed in New York, at the Pentagon and in a field in Pennsylvania.

For the people born that day, it also marks what began.

Before it belonged to history, Sept. 11 was already a happy day in Denaro’s family. She was born just outside Boston on her parents’ second wedding anniversary.

Her father was in the fire academy at the time. Hours after his daughter arrived, New York needed firefighters.

“So he went down and met with his chief, who luckily was like, ‘You just had a baby a few hours ago. You’re not going anywhere. Like, go back to the hospital,’” Denaro said.

The family spent the rest of the day in that hospital room, watching the attacks unfold on television.

Denaro can’t point to the moment she understood what her birthday meant to everyone else. She only knew, from the time she was small, that it meant something.

“Even if I didn’t necessarily know why,” she said. “I knew when I would tell people, adults specifically, when my birthday was, I knew I was going to get a reaction of some sort, like they were going to gasp or be shocked.”

For Megan Blasko, life and loss are tangled even tighter.

She arrived three weeks early, just outside New York City. Her father, a New York Police Department officer, was supposed to be stationed next to the twin towers that morning.

Instead, he was with his newborn daughter.

When people react to her birthday, Blasko, who now lives with her family in Conway, South Carolina, has an answer ready.

“I tell them that I have a good reason to celebrate it because my dad was supposed to be there and me being born three weeks early is one of the reasons he wasn’t,” she said. “I get to have him my whole life, where there was a lot of people who didn’t get that.”

But the day has never been only about what her family was spared. Her uncle, a first responder, survived the attacks and later died of cancer connected to his time at ground zero.

Her birthday holds both: who her family kept, and who it lost.

Jeni Driscoll remembers a Sept. 11 that Denaro and Blasko never knew, when it was simply her birthday.

She was 37 that Tuesday morning when her husband’s assistant called him at work.

“She said, ‘Turn the TV on right now.’ And that’s what we saw,” Driscoll said. “And so throughout the day, of course, I was watching TV all day long with friends and family, and I didn’t want anybody to know it was my birthday.”

In the 25 birthdays since, she has tried to make room for both.

“Before 9/11, my birthday was Sept. 11. That’s it, a quiet day,” she said. “And now it’s known as 9/11, synonymous with terrorism. And that doesn’t feel good for sure. But it also makes me reflect on all the people that lost their lives that day on 9/11 and how lucky and how fortunate I am to be celebrating my life on that day.”

Asked what advice she would give someone born on 9/11, Driscoll kept it simple.

“To never forget, to learn about what happened, and to let it be a reminder, as I do, that life is so precious, it’s fragile,” she said.

One woman remembers the birthday before. Two have only ever known the birthday after. All three have spent a quarter-century learning to hold two truths at once: that the day took so much, and that it is still worth celebrating.