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.”