FC Union Berlin are not your ordinary club. The small team that lived in the shadow of the Stasi-sponsored Dynamo Berlin for so many years are now thriving in the German second division with remarkable fans who once saved their club by giving en masse blood transfusions and, in 2008, worked for 140,000 hours to rebuild their stadium.

And on Sunday they will carry out what must surely be the best Christmas tradition in the world of football when they meet up at the stadium to sing Christmas carols for 90 minutes. And there's not even a game on. When the tradition started in 2003, 89 people turned up. Last year 17,000 came.

The level of commitment from the fans is extraordinary, and the club are far, far away from the multibillionaire-owned clubs such as Manchester City and Chelsea. And yet they could be playing in the Bundesliga next season. They entered the winter break seventh in the second Bundesliga.

Unification has not been kind to the clubs from the former GDR. Several clubs that used to be so successful never found a way to make it in the new world of free market and capitalism. Since a few years back there is not even a club from the east in the Bundesliga.

That is why so much hope is attached to Union. The club's economy is healthy and their league position is at an all-time high.

But it wasn't always like that. The defender Rolf Weber played 206 league games for Union in the first and the second GDR league between 1970 and 1981. When I asked him to explain what is so special about the club he answered: "We lost more games than we won, but the fans never let us down. The atmosphere always was amazing."

In fact, Union have a bigger track record of economical crises than winning trophies. And the fans are always there to help. In 2004 the club urgently needed about £1.5m to avoid bankruptcy. The supporters then organised a campaign named "Bleed for Union" where fans gave blood and forwarded the reimbursement to Union Berlin.

The close relation between club and fans is well documented. The president, Dirk Zingler, is a supporter who has a big understanding of what the fans think and what they want. In an interview with football magazine 11Freunde he was asked what he thought of the fans' criticism of the police's conduct at an away game in Frankfurt. "No one has to tell me what went on there," he said. "I was one of the 1,500 fans there."

In 2008, when Union had to modernise their stadium, Stadion an der alten Försterei, Zingler quickly established that the club would not be able to afford it without a great contribution from the fans.

No problem. With the help from almost 2,400 volunteers the stadium was rebuilt in 300 days. Together they worked an extraordinary 140,000 hours without being paid and Union now have a modern stadium, which still has 80% standing.

Also, the club are quite allergic to transfer fees. Until two years ago there was a strict policy of not paying any transfer fees. That had been ongoing for five years. There has been a relaxation of that policy as the club have become stronger but their record transfer is still the modest £400,000 they paid for the Brazilian striker Silvio when he arrived from the Swiss club Lausanne in the summer of 2011.

FC Union Berlin have always been outsiders in German professional football. It's not a coincidence that the famous punk singer Nina Hagen sings the club hymn that is played in the stadium before every game.

Before 1989 Dynamo Berlin, the fiercest rivals, belonged to the Stasi – ministry for state security – and were in many ways a symbol for the hated GDR regime. Union Berlin were the free football club, without any connections to army, police or state. Stadion an der alten Försterei (Stadium near the old forester's house) developed into a meeting point for regime critics. When Union had a free-kick, the spectators used to shout "Die Mauer muss weg" (the wall has to fall). It went so far, in fact, that people went to Union games without even being interested in football.

In 1989, the wall fell, and the hated club of Dynamo Berlin did as well. BFC Dynamo, who won 10 league titles during their glory days, are nowadays a mediocre team in the fifth division.

As mentioned, the 2 Bundesliga has entered its winter break but on Sunday the fans will gather to sing for 90 minutes one last time before Christmas. The stadium will be packed again. Not for a game but for the single purpose of drinking Glühwein (mulled wine) and singing Christmas carols.

This year is the 10th anniversary of this peculiar but warming tradition. Torsten Eisenbeiser, who came up with the idea, says: "After a couple of bad games we, who always stand together during the games, just went home without wishing each other merry Christmas. I suggested to one friend that we should get together in the stadium the day before Christmas Eve and sing some Christmas songs."

In the end 89 people showed up for this underground event. Officially the stadium was closed. Through word of mouth, however, the event has got bigger and bigger. In the third year it became an official event and last year more than 17,000 people made their way to the stadium. Not only from Berlin, but from several other parts of Germany as well as other countries.

Over the years the Union Christmas singing has changed shape. Now it goes on for 90 minutes (plus some stoppage time) and every visitor is given a candle and a booklet with all the songs.

Even some of the players and their families attend. The defender Christian Stuff is one of them and for him this tradition is proof that Union Berlin are a special club.

"As a player you appreciate it here," he says. "Not only the atmosphere but also the understanding from the fans. They don't demand that we should win every time, just that we always are trying our best. The Christmas singing is extraordinary. Just to see all the candles is very cool."

It has also transpired that fans from other clubs also take part. As early as the second year, a Hertha Berlin member card was found in a wallet that had been left behind.

Eisenbeiser says: "We just wish each other merry Christmas – it doesn't matter which team you are supporting. No one leaves the stadium as a loser."

FC Union Berlin facts

Founded 1966, originally as FC Olympia Oberschöneweide 1906

Members 10,500.

Stadium Stadion an der alten Försterei, in Köpenick, south-east Berlin

Capacity 16,750.

Honours East German cup winners 1968 (they reached the German Cup final in 2001 but lost against Schalke 04)

Cult player The captain, Torsten Mattuschka. Joined the club in the fourth league in 2006

Nickname Eisern Union (Iron Union)

Union blog in English unionberlinman.blogspot.se