How woman who lost mother-in-law in Pittsburgh synagogue shooting aims to fight anti-Semitism during Passover

As the Jewish holiday of Passover begins Friday night, families and friends will gather at Seder -- an orderly, annual dinner where they eat, drink, sing and re-tell the story of the enslaved Jews' escape from Egypt.

For 12 years, Marnie Fienberg hosted her family's Seder with her mother-in-law, Joyce Fienberg.

But that tradition has ended.

Joyce Fienberg, a 75-year-old grandmother of six and former researcher at the University of Pittsburgh, was among the 11 worshipers shot dead at Pittsburgh's Tree of Life synagogue on Oct. 27, 2018.

"My mother-in-law was one of the kindest humans I've ever met," Marnie Fienberg told ABC News in October. "If you knew her for five minutes, if you knew her for 20 years, you felt exactly the same way."

The Anti-Defamation League has called the Pittsburgh synagogue massacre the worst anti-Semitic attack in U.S. history.

In the wake of her grief, Marnie Fienberg was desperate to put her energy into a positive, grassroots way to fight anti-Semitism in her mother-in-law's honor.

And Friday night, her plans will come to fruition.

Marnie Fienberg founded "2 for Seder," an initiative that encourages Jews to invite two non-Jewish friends to a Seder dinner.

Now 910 Seders across North America are set to participate this weekend, she told ABC News on Friday.

"Whenever I go to someone's house or I go into a new situation, it's always nicest when I bring a friend or I bring my spouse," she said of the "plus-one"-style invite. "You're going to be coming into a family situation and I want the guests to be as comfortable as possible."

Marnie Fienberg on her website calls "2 for Seder" a "small step toward fighting anti-Semitism by addressing the 'mystery' of being Jewish, the 'Other' in a society filled with many wonderful and diverse cultures from across the world."

"I want to make sure in this country we have a positive dialogue going on about the different religions, the different races, the different lifestyles that we have," she said.

Anti-Semitic threats and acts had been on a decline in the U.S. until three or four years ago, according to John Cohen, a former acting undersecretary at the Department of Homeland Security (DHS) and current ABC News contributor.

Of the 1,749 victims of anti-religious hate crimes in 2017, 58.1 percent were victims of crimes motivated by the perpetrators' anti-Jewish bias, according to statistics from the FBI.

An open and welcoming Seder initiative couldn't be more fitting as a way to honor the woman who was just as meticulous a host as she was a researcher, said Marnie Fienberg.

"If you were coming to her home [Joyce] would do anything to make you feel completely and utterly comfortable," Marnie Fienberg said. "That didn't matter if it was an allergy you had, or if you had a religious restriction, or if you were coming from another culture... she would research it, make sure she was making you comfortable...and she made it all look effortless, even though she would spend hours and hours and hours prepping."

After Passover ends, Marnie Fienberg said she plans to invest her time in other grassroots initiatives.

"We just have such a positive response. And I think there's a lot of energy out here and people want to put that positive dialogue out into the discussion, and I want to help facilitate that," she said. "So this is, I hope, just the beginning."

As for Fienberg's Seder this weekend, she said, "Joyce's presence will absolutely be with us."