Nadya Chernyakov is 15 and a Jew by birth. But today, for the first time in her life, she is observing Yom Kippur, the most solemn of the Jewish holy days.

Chernyakov got her first real taste of religious freedom -- and her Jewish heritage -- 10 months ago when she immigrated here from the Soviet Union. In that country, she said sadly, "there is no religion."

The Chernyakovs, along with many of the 165,000 Jews in the Washington metropolitan area, began fasting last night to observe Yom Kippur. They will send much of their day today praying in their synagogue.

"We never celebrated religious holidays there [in the Soviet Union]," said Chernyakov, a pretty, girl-woman with huge expressive brown eyes. "We weren't allowed to. I didn't even know about them.

"My mother told me I was Jewish but I never knew what that meant."

She knew more than some children of Russian Jews, however. Her parents had kept a few illegal religious books around the house and she had picked up a few words of Yiddish, listening to her grandparents speak it occasionally around their home.

But now Nadya, a student at the Hebrew Academy of Greater Washington in Silver Spring, is the one to teach her parents, trying to help them understand Yom Kippur, which her mother, Vera, said is only observed by very old people in the Soviet Union.

Chernyakov and Tanya Feld, 14, her best friend at school, sat in the cafeteria this week sharing painful memories of how they had kept their religion secret from their Russian friends and teachers.

"We didn't really know what 'being Jewish' meant," said Feld, "but we didn't like it much."

"Nobody likes Jews in Russia," Chernyakov said. "If you're Jewish you don't tell anybody. If you do, they'll call you names, laugh at you and tell real bad lies about you. . . . But nothing really terrible happens."

When her classmates found out Feld was leaving Leningrad three years ago, they wanted to know why. "So I stopped going to school the last month," she said in a whisper, her eyes lowered.

"But it's really good in America," she said. "You don't have to hide your religion. . . ."

Now Feld and Chernyakov are mastering Hebrew, as well as their religious history and traditions. "We didn't know that being Jewish meant that much," said Feld.

And Nadya is taking those lessons home. "We only know a little about the holy day but we read and try to follow customs [now]," said Vera Chernyakov.

Like other observant Jews, Mrs. Chernyakov said she plans to spend the day in introspection. "I like having this special time to think over your life and your relationships with relatives and friends," she said. As Nadya added, "It's nice to know where you come from."