In the darkness of an Upper West Side concert hall last weekend, 150 audience members holding twinkling plastic candles sang and swayed to celebrate reason and the season. Snow fell with abandon outside.

“We are not alone,” a humanist rock band crooned in a call and response.

“I wanted a holiday that made us feel connected, and feel connected to the world,” Raymond Arnold, the M.C., said at the start of the show he created, “Brighter Than Today: A Secular Solstice.”

Mr. Arnold, 27, a self-described “agnostic-atheist-humanist” who grew up in Poughkeepsie, N.Y., told sardonic sermon-like stories to explain scientific developments since Stonehenge.

Then he invited the audience to sing a Christmas carol. “Some of you might be like, ‘I came to a secular solstice, what up?’ ” Mr. Arnold said, drawing laughs. He explained that “Do You Hear What I Hear?” did not mention Jesus Christ and could refer instead to the birth of an idea. He was going for “a sense of transcendence,” he said. It felt a little like church.

This month, at the height of the religious season, New York secular groups — atheists and agnostics; skeptics and rationalists; humanists, freethinkers and nontheists — offer potluck dinners, solstice celebrations, discussions about the tyranny of the holidays, and even special karaoke nights. The underlying goal is to replace the stereotype of the “angry atheist” as a Scrooge with the notion that the godless also want to have fun at this time of year.

And why not?

On view at Mr. Arnold’s event, which was funded by a Kickstarter campaign that raised $8,330, was a distinctly millennial brand of nonbelief. Attendees, mostly in their 20s and 30s, were of various sexual orientations and varied backgrounds. One was a raw-vegan science teacher who grew up going to a yeshiva; another, part of a group from Boston, was wearing Google glasses.

The crowd fit the recent explosion of secularism: One-fifth of Americans do not identify with a religion, and a third of those under 30 have no religious affiliation, according to a 2012 study by the Pew Research Center’s Forum on Religion and Public Life. There’s even the hit indie song “Unbelievers” by Vampire Weekend for doubters to make their anthem.

“We’re trying to ride the crest of that wave and bring a more open, vocal and proud face of atheism,” said Michael Dorian, 49, New York’s regional director for the American Atheists. Mr. Dorian runs a musical variety show in New York, the “Godless Revival,” which planned a solstice party on Dec. 22 at Connolly’s Pub in Midtown. After Mr. Dorian’s afternoon shows, which do not encourage singalongs, the audience is invited to stay for “The Afterlife,” an informal social hour.

“We’re fun-loving,” Mr. Dorian said. “We want to show we’re not a bunch of outcast, alienated, creepy folks.”

But at times, even without God, organized atheism can look a lot like organized religion — complete with the schisms. The Godless Revival split from a group called Sunday Assembly, a “godless congregation” started by two British comedians, Pippa Evans and Sanderson Jones, that began barnstorming across the United States this year. After helping to organize Sunday Assemblies, Mr. Dorian said he and two colleagues on the steering committee clashed with the British founders over the churchlike feeling and the rule against using the word “atheist.”

Now the imported Sunday Assembly is being hosted by the New York Society for Ethical Culture, which considers itself a nontheist religion. It, too, is seeking to bring in a younger demographic.

Reasonable New York, a consortium of secular groups, offers a road map to the local scene on its website calendar and was sponsoring its own party on the Dec. 21 solstice at a downtown nightclub. Conspicuously absent from the coalition, though, is one of the most established groups, the New York City Atheists.

The organization’s president, Kenneth Bronstein, a retiree, and its vice president, Jane Everhart — both declined to give their ages — revived the moribund group in the last decade, with aggressive canvassing at Columbus Circle and their commitment to the separation of church and state. Many of their events tend to attract older participants.

“We feel we’re much more effective as a stand-alone organization,” Mr. Bronstein said, explaining why his group had declined to join the coalition.

“If we worked, say, with humanist groups, we wouldn’t get things done because of a difference in focus.”

Mr. Dorian praised the efforts of Mr. Bronstein and Ms. Everhart, but added that it was time to cultivate “a broader appeal to younger people who don’t feel such a sense of urgency to problems of atheism and religion.” He was partnering with the group to throw the solstice party.

The Gotham Atheists, a group that also leans older, itself split off from the New York City Atheists, said its founder, Rich Sander, 50. Originally, his group was called Richie’s List, since he provided information about other groups’ events in the area.

Mr. Sander started a Drinking With Atheists group that gathers every Friday at a Midtown bar, but on Dec. 13 he hosted a potluck dinner and holiday discussion at his apartment instead. The six attendees recalled the origin of evergreen trees as a pagan symbol, and how the winter solstice was the first holiday celebrated at this time of year.

“For us as atheists, we can look at the holidays and — I wouldn’t say co-opt, but take back,” Mr. Sander said. “We want to enjoy this part of the season.”

Even amid the different social gatherings, some members recognized the inherent difficulty in maintaining cohesion, since they are bound together only by their rejection of mainstream religion.

“With secular groups, it’s hard getting people to join because it’s like herding cats,” said Tiffany Crespi, 36, a third-generation atheist and a member of the New York chapter of the Center for Inquiry, a group focused on science, reason and humanistic values. “A lot of people have reasons for becoming atheists. They get sick of church and religion and organizations in general and they don’t want to join another group.”

One recent Monday night, Ms. Crespi was one of 10 young men and women from the Center for Inquiry who stood freezing on a Times Square sidewalk for 45 minutes, waiting for 15 seconds of inspiration. When the video billboard sponsored by the Center for Inquiry finally appeared on 42nd Street, pronouncing that millions of Americans are living happily without religion, the chilled audience whooped, clapped and hastily took a picture.

Over pizza nearby, they debated the subtle tone of the billboard compared with that of the more controversial video billboard that was on Broadway (and is now across from Penn Station) sponsored by the American Atheists: “Who Needs Christ During Christmas? Nobody.” The ad ends by urging people to celebrate the true meaning of “Xmas,” with flashing words like charity, family and food.

“They have their style, and I like it,” Georgina Capetillo, 24, said of the American Atheists’ national campaign.

“But the thing is,” said Gary Gibson, 27, “coming from one of the most distrusted groups in the country you might want to ease up a little bit.”

“But you don’t,” Ms. Capetillo shot back.

“They’ve already made their point about being caustically aggressive,” said Michael Nam, 37. “The holidays, it’s a good time to lay aside that confrontation.”

Soon, the members did just that, as they headed to the East Village for some karaoke at a monthly event called Skeptics on the Mic. Mr. Gibson sang a rap song from Outkast. Alexa Blumenstock, 19, led a spirited rendition of “Under Pressure” by Queen and David Bowie.

The season, reflected another member half-jokingly, does not have to be stressful for nonbelievers, even when they return to their religious families.

“In my opinion, you never need any reason to get drunk with your family during the holidays,” said Alexander Woodman, 20, before correcting himself, secularly: “during the winter.”