The temperature was below freezing and dropping rapidly along the main road of Rhinebeck—a town in New York State’s Dutchess County, 102 miles north of New York City—as the sun went down before Shabbat Chanukah four years ago.

Inside their old Volvo, Rabbi Yosef and Rachel Rosenberg were lost. “Very, very ferblunget,” he says.

The couple, who had already lit theirMinutes away from sundown, there was no way for them to get there before Shabbat Chanukah candles at home, was headed to the home of Rabbi Yitzchak Hecht, the Chabad emissary in Kingston, when their GPS guided them to the wrong side of the Hudson River.

Minutes away from sundown, there was no time for them to get to the Hecht home before Shabbat. Then they thought of another nearby Hecht. Rabbi Yitzchak Hecht’s brother Hanoch was the Chabad shaliach (emissary) in Rhinebeck. The Rosenbergs had spent Sukkot with the Rhinebeck Hechts a few years earlier. They decided they would go there for Shabbat.

Unannounced—they didn’t have a cell phone—they drove to the Hecht home where they had stayed a few years earlier. It was empty. The Rosenbergs were not aware that the Hechts had moved.

Now it was time to walk; it was so close to candle-lighting. Rabbi Rosenberg, then 70 years old, had been ordained by the Lubavitch rabbinical school in Morristown, N.J., and had worked as a substance-abuse counselor in Los Angeles. He parked the couple’s car in a small shopping area, emptied his pockets, put their valuables in the trunk, took out his walker (which he uses because of neuropathy and a long-standing back problem) and davened Mincha before it got dark.

Then they found a non-Jewish employee at a health food store and explained everything about being Orthodox Jews and being unable to use their car on Shabbat. They handed him their car key, to be picked up after Shabbat. And the employee found the Hechts’ correct address in a phone book and gave the visitors directions.

Then they set out.

Why didn’t Rabbi Rosenberg rule that exposure to the frigid weather could be a case of pikuach nefesh (saving a life), as it possibly endangered his and his wife’s health? Why didn’t he decide that it would make sense to drive to the Hechts?

“We wanted to walk. We knew we had to,” Rabbi Rosenberg, who grew up in Newark, told me one sweltering day this summer. We were sitting on the back porch of the Rosenbergs’ home in Mountain Dale, N.Y., 39 miles from Kingston, in the shadow of towering white pine trees. “We knew we weren’t going to freeze,” he said. They risked no other danger, the rabbi said. “It wasn’t wilderness,” no wild animals or brigands roaming around.

Neither Rabbi Rosenberg nor his wife, who was from the Bronx, was raised in a traditional, Shabbat-observant home, but violating Shabbat because of a doubt about a permitted exemption was out of the question, they said.

“Nobody was out,” Mrs. Rosenberg said. “It was too cold.”

Their winter stroll seemed a long way off as they related their story. But for the couple, the memory was as fresh as the vegetables they pick from the garden on their large grounds.

The wind that December day was blowing in their faces, they recalled. They could see their breath against the dark sky. Balancing themselves, especially while leaning on a walker, was treacherous. The rabbi, not prepared for his impromptu hike, was wearing sneakers. His wife, a retired nurse and master gardener, then 63, had boots on. Fortunately, both had down parkas and gloves.

“It was bitter, bitter cold—brutal,” Rabbi Rosenberg said. The gloves didn’t help much. There was snow on the sidewalk that went over their ankles, and ice. “The street wasn’t any better,” said Mrs. Rosenberg. “We walked on the sidewalk when it was good. We took the street when it was better.

“I was crying a little bit,” she said, from the cold. “I prayed to G‑d to get us there.”

Rabbi Rosenberg walked in front of his wife to block the wind.

The Hechts, who had no idea that theWe walked on the sidewalk when it was good. We took the street when it was better Rosenbergs were on the way, lived a mile or two away.

The Rosenbergs walked for 40 minutes, maybe an hour, they said. Along the way, they sang nigunim (chassidic melodies). “Any song we could think of,” Rabbi Rosenberg said. “We tried to tell some jokes.”

“We knew we were making a memory,” Mrs. Rosenberg said.

Meanwhile, the Hechts, who have lived in Rhinebeck for a decade, were in Shabbat mode. Rabbi Hecht had davened Mincha and Kabbalat Shabbat, and the family was gathering around the dining room table.

“What, no guests?” asked Mendel, their son, then six. He liked the guests who joined the family for Shabbat meals every week.

Not tonight, Rabbi Hecht explained. It’s too cold outside. No one showed up for davening; no one’s coming for dinner.

Mendel wasn’t pleased, but he accepted his father's explanation.

Shalom Aleichem and Eishet Chayil had been sung, and Rabbi Hecht was about to make Kiddush.

He took the cup in his hands, and was about to say, "Baruch..."

“Then,” says Tzivie Hecht, Rabbi Hecht’s wife and partner at the Rhinebeck Jewish Center, “there was a knock at the door.”