Christian Arbeit, a press officer for the club, joked that the renown of Union Berlin’s Christmas party had overshadowed the fame of the team itself. Mr. Arbeit, who served as the M.C. on Friday night (and also played a few songs on the trumpet in an ensemble that featured his mother on the clarinet and his father on the trombone), said the aim was to retain the intimate feeling of the event, despite the enormous crowd.
Stadion An der Alten Försterei, the team’s home, sits on the edge of a forest in the quiet neighborhood. Somehow, in spite of the club’s modest ambitions and working-class sincerity — or perhaps because of these things — it has become a fashionable team to follow, in Berlin and elsewhere.
On Friday, the happy-go-lucky club with the quaint stadium was suddenly tasked in a newly tense city with hosting an event that at times took on the feeling of a memorial service. In the wake of tragedy, sports teams often find themselves in this role: Two nights earlier at the Olympiastadion, an announced crowd of 31,912 had observed a minute of silence before Hertha B.S.C., the local first-division team, played its final home match before the Bundesliga’s winter break. A large banner in the stands there read, in German, “Stay strong, Berlin.”
Tara Türk, 23, who was attending the Union’s event for the third time, noted the emotional atmosphere. “This is our best chance to show we’re not just in fear, sitting at home,” Ms. Türk said. “You try to have fun, make the best of it.”
Club officials met on Tuesday morning, the day after the truck attack on the Christmas market, and discussed, briefly, whether they should cancel this year’s event. Instead they decided that it would go on, with increased security from the Berlin police and small measures to monitor and protect the large crowd, like more lights around the stadium.
Mr. Arbeit said he was aware that some fans were anxious about attending, and that some had even given up their tickets. But he also said the club recognized its role in the city’s collective healing after the attack. The goal for the event, he said, was to carry on as usual — that is, festively.
“We didn’t want to make it a sad evening,” Mr. Arbeit said. “We wanted to make it a night of hope, being together, singing and empowerment.”
Mr. Arbeit began the night with a request for a moment of silence to honor those who were killed or injured in Monday’s attack. People in the crowd lifted their candles high above their heads during the 30 seconds of quiet.
Later in the night, as Peter Müller, a pastor from a local church, stood on stage to tell the story of Christmas, he wove references to the attack into his message, calling for unity in a period of strife.
But otherwise, the night’s program was a celebratory affair. Fans engaged in impromptu, call-and-response chanting, their breath visible in the 37-degree night.
Candles and lyrics were passed out to each attendee, and traditional carols like “O Tannenbaum” were mixed with medleys of game-day chants. The club’s costumed mascot shimmied around the stage, helping to keep time.
“This week we’ve been sad, nervous, and we can’t believe what happened,” said Marina Müller, 26, of Eichwalde. “But I think you cannot change your life. We wanted to share this feeling with the many people here, to sing, and have a merry Christmas.”
Union Berlin’s welcoming reputation only added to the atmosphere. In the past two decades, the club’s efforts to nurture a feeling of old-school romanticism, and local-club familiarity, have earned it a measure of fame outside Berlin.
In 2008, for example, when the club determined that the stadium required extensive renovations, more than 2,000 fans volunteered manual labor over the year that followed to rebuild its grandstands.
In 2014, the team invited fans to turn the stadium into a “living room” to watch World Cup matches on a giant video screen. Approximately 800 sofas were lugged from nearby homes and arrayed on the field alongside side tables and lamps, and pictures of the oddball scene were shared around the world.
Those endearing episodes have hinted at a level of reciprocal warmth unusual among a professional sports team and its surrounding community.
It was on display Friday night, in bright lights, and in full voice.