Rabbi Yaakov Saacks started a congregation in a rented house in Dix Hills. His office was in the boiler room in the basement.

Now, as Jews around the world on Friday mark the start of the holiest day of the year — Yom Kippur — Saacks has reason to be grateful. His congregation at Chai Center of Dix Hills is starting its 25th year, and it has moved from the rented house to a full-fledged synagogue on Vanderbilt Parkway.

“I’m grateful to the community at large,” Saacks said. “The community really saw, perhaps with even better clarity, the vision that I conveyed but didn’t always see fully myself.”

The synagogue, part of the Orthodox Chabad Lubavitch movement, has grown from about 20 participants in its first High Holy Days in the rented house to 1,100 during Rosh Hashanah services last week, Saacks said.

Yom Kippur, also known as the Day of Atonement, is the most solemn and important day of the year for Jews. Starting at sunset Friday, the faithful will abstain from eating, drinking and other comforts for 25 hours.

They will spend the first evening and the next day in prayer and intense self-reflection, until the stars come out Saturday, about an hour after sunset.

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Yom Kippur culminates the High Holy Days that began with Rosh Hashanah — the Jewish New Year — on Sept. 20 at sunset. It concludes a monthlong process in which observant Jews focus on how they can improve their relationships with others.

Saacks had just finished his service as campus rabbi at Stony Brook University when he decided to start his own congregation. He rented the house and took out newspaper ads to attract followers.

After a few years, the growing congregation began holding services in a small, rundown cottage off the parkway. That was torn down in favor of the synagogue in 2001. A decade later, the synagogue doubled in size, to 25,000 square feet.

On Yom Kippur, Saacks will reflect on how far the congregation has come.

“Yom Kippur is not so much repentance, but more resolve,” he said. “I think that’s what an anniversary is — you look back and say, ‘What is it that I have to do in the future?’ ”