Imagine the darkest time of year in Sweden, when the sun never climbs above the horizon in the northernmost part of the country, and in Stockholm, the capital, there’s a scant six hours of daylight. Every day is shorter and gloomier than the last until the nadir — the darkest day of the year — when a young woman appears dressed in a white gown with a blazing crown of candles on her head, singing a familiar song and spreading warmth and light on a frigid winter morning.

That’s the traditional Swedish tale of Lucia, or, as she is sometimes referred to, St. Lucia, a mythic figure who leads candlelit processions all over Sweden on Dec. 13. Luciadagen, or Lucia Day, is one of the most culturally significant holidays in Sweden.

On that day, schoolchildren wear costumes and sing “Sankta Lucia,” the traditional Lucia song, for crowds of teary-eyed parents: “St. Lucia, emblem of lightness, spread in our winter night, the sheen of your brightness,” begins one of the most famous versions. Workplaces hire local choirs to perform for employees, and churches and cultural institutions host processions on the day that many Swedes consider the true beginning of the Christmas season.

“We are fully booked on the 13th of December,” said Ulrika Nordlander, 41, a longtime member of the Stockholm University choir. “From morning to night, we sing in Lucia processions across all of Stockholm.”

Last year, Ms. Nordlander led the procession at one of the city’s most atmospheric locales: Seglora kyrka, an 18th-century wooden church at the open-air museum Skansen.

“It’s an honor to lead the choir and feel that you’re coming in and spreading light and joy,” Ms. Nordlander said. “Especially how it brings light into the darkness, it’s really beautiful to see.”

In addition to celebrations in Sweden, there are Lucia events in U.S. cities like Minneapolis, Philadelphia and New York — places where there are strong Swedish communities. Wherever they are held, it is customary after each procession to gather for coffee, gingerbread cookies and Lucia buns, sweet S-shaped saffron buns dotted with raisins that are known as lussekatter, lussebullar or julgaltar, depending on whom you ask.

A Sicilian saint in Sweden?

But how did Lucia, originally a Sicilian saint from Syracuse, who was supposedly martyred there in the fourth century, become a cultural pillar on the opposite end of the continent, in the historically Lutheran country of Sweden?

“Where she came from nobody really knows,” said Jonas Engman, an ethnologist and scholar specializing in Swedish traditions and folklore at the Nordiska Museet in Stockholm, referring to the Swedish version of Lucia.

Pastoral celebrations around the winter solstice, similar to the midsummer festivities around the summer solstice, are the likely origins, he said. And the reason the holiday falls on Dec. 13, rather than the true winter solstice (around Dec. 21), is because of a shift from the Julian calendar to the Gregorian calendar in the 18th century.

The widespread celebration of Lucia, as the holiday is observed today, only began in the 20th century, Dr. Engman said. And it really took off after the Stockholms Dagblad newspaper started a competition to select Stockholm’s Lucia in 1927.

“The first Lucia chosen in 1927 was actually not blond. She was dark-haired,” Dr. Engman said. But in the decades that followed, Lucia became synonymous with a young, blonde Swedish woman. (Anyone familiar with the original American Girl dolls will recognize this stereotype as Kirsten, the Swedish pioneer with blonde braids and her own little Lucia outfit.)

These days, anyone can play the role of Lucia regardless of hair color, skin color, nationality or gender. Even a bare-chested professional soccer player can don the crown of candles, as the Swedish sports icon Zlatan Ibrahimović once did in a video to affirm that men can also be Lucia.

As communities of Swedes have settled around the world, so, too, have their traditions spread, including the joyous celebration of light on Luciadagen. My mother, who emigrated from Sweden in 1969, has long been an active member of the American Swedish Historical Museum in Philadelphia and for me, a Swedish-American kid in the 1980s and ’90s, the museum’s annual Lucia celebrations were a highlight of the year. The entire 20,000-square-foot museum was kitted out for the occasion, with galleries transformed into Christmas markets selling Swedish handicrafts and traditional baked goods, and the grand central staircase serving as the stage for Lucia processions.

Every performance would begin with a singalong of Swedish Christmas songs — “Nu är det jul igen,” “Stilla natt,” “Nu tändas tusen juleljus” (“Now it is Christmas Again,” “Silent Night,” “Now a Thousand Christmas Candles Are Lit”) — and then the youngest children would appear dressed as tomtar, or gnomes, singing and clapping in cute red costumes. Next were the pepparkaksgubbar (gingerbread men), slightly older kids singing about their off-kilter hats. After an interlude with folk dancers, it was time for stjärngossarna, the solemn star boys, in white gowns and cone-shaped hats, who were often played by girls (myself included). Finally the lights would dim for the grand finale: the Lucia procession.

The year I was invited to be Lucia at the museum, I was 14. Waiting in an antechamber off the museum’s staircase, someone lit the candles — battery-powered, for safety reasons — in the crown atop my head as the singing began. There were about a dozen girls as Lucia attendants, each in a white gown with a red sash and a wreath on her head, carrying a single candle and singing “Sankta Lucia,” which many Swedes know by heart (and which was originally adapted from a Neapolitan folk song, curiously). When the attendants reached their positions along the edge of the stairs, I stepped out to sing a solo verse in front of the hushed crowd. It was a moment I’d dreamed of for as long as I could remember, since I was a tiny tomte sitting cross-legged on the floor, entranced by a luminous Lucia striding down the stairs.

These days, you’ll find me in the audience, wherever I can find a nearby procession, whether it’s a local choir parading through my Stockholm co-working space or, this year, at the Lucia concert at Svenska Kyrkan in Manhattan. I’ll be quietly humming “Sankta Lucia” to myself, and taking in the light.

Where to see a Lucia procession

Many churches and cultural institutions in Stockholm have Lucia processions on Dec. 13, as well as on the weekend before the holiday, including Seglora kyrka at Skansen and Storkyrkan in Gamla Stan.

Swedish communities in the United States often organize Lucia celebrations, including at the Gloria Dei Old Swedes’ Episcopal Church in Philadelphia, at Svenska Kyrkan in New York, at the American Swedish Institute in Minneapolis and at the Swedish American Museum in Chicago.

If there’s no Lucia celebration nearby, tune in to Luciamorgon, a special program featuring a Swedish youth choir that is broadcast on the morning of Dec. 13 on SVT, Sveriges Television, and can be streamed on SVTPlay.

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