For Ryan O’Day, it took one book to change his perspective.

The Glen Head teen’s mother encouraged him to read “Etched in Sand,” Regina Calcaterra’s memoir about surviving a broken family and foster care on Long Island, when it showed up last year on the North Shore High School summer reading list. O’Day’s mother was hoping it would have the same impact on her son as it did on her book club when they read it.

For O’Day, what started as an assignment became a passion to help older foster kids, who are less likely to be adopted and often become homeless once they age out of the system at 18. O’Day is seeking to change that with Teens Gotta Believe, an awareness event and fundraiser scheduled for Saturday.

“I’m not comfortable sitting here and letting that happen,” O’Day, 17, said of the current situation. “I think, ‘What if that was me or a friend?’ ”

The event will feature a basketball tournament, track competition and an author talk and book signing with Calcaterra at North Shore, where O’Day is a junior.

O’Day said he’s expecting about 100 people to turn out. Donations from registration and collected at the event will support You Gotta Believe!, a Brooklyn-based organization dedicated to finding families for older foster kids.

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O’Day did much of the planning himself, relying on his experiences participating in Relay For Life, which benefits the American Cancer Society, and Hoops for Harrison, for the Crohn’s & Colitis Foundation of America.

His mother, Jillian O’Day, said she sends a few organizational emails while he’s at school. “As a mom on the North Shore of Long Island, we’re really fortunate,” she said. “For him to realize how fortunate he is, is huge to me.”

In 2014, the state Office of Children and Family Services reported 207 kids in foster care in Nassau County and 677 in Suffolk. About half in both counties were older than 10.

Nationally, about 50 percent of young adults who age out of the system become homeless, according to You Gotta Believe!

“When did you stop needing your parents? That’s what we always talk about,” said You Gotta Believe! board president Gretchen Beidl. “You need support, you need a community, you need a family to get through life, we all need that.”

The statistics shocked O’Day. After he finished “Etched in Sand,” he reached out to Calcaterra, asking how he could help.

“It’s a gift I never expected,” said Calcaterra, who lives in New Suffolk.

Calcaterra, an attorney, and her four siblings grew up in and out of homelessness with absent fathers and an abusive, neglectful mother. She won emancipation from her mother at 14 and was funneled into the foster care system in Suffolk County, fighting to keep her siblings together. Calcaterra eventually went to college and served as Suffolk County deputy executive and as executive director of Gov. Andrew M. Cuomo’s Moreland Commission investigating public corruption.

Readers often reach out to her and some are moved to adopt, but Calcaterra said O’Day’s goal of raising awareness is far-reaching. She put him in touch with You Gotta Believe!, where she serves on the board.

“He just called us and said I’m doing this, and we said, ‘OK, that’s awesome, what can we do to help,’ ” Beidl said.

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Even those who can’t adopt a child can still offer support, as Calcaterra said she received when her friends’ mothers would give her a warm meal and wash her clothes. O’Day said he wants his community to know there is always a way to help improve the quality of life for foster kids, whether it’s seeking policy changes or just offering a hot meal.

“The more politicians and even just local people that are aware of it, the better it is,” O’Day said.

Teens Gotta Believe will run from 1 to 6 p.m. Saturday at North Shore High School in Glen Head. More information and registration forms are available at teensgottabelieve.org.