The story of how Union started this tradition, this celebration, is worth remembering, for it sums up much about them and the relationship to their fans. It had begun - as is often the way of these things - with the Unioner enduring a torturous season, watching loss after loss, and sitting in the relegation places of the 2. Liga at Christmas.

A few fans (including Torsten Eisenbeiser who would organise the event for many years after, and the sadly missed Tino Czerwinski) decided that they should shrug all this off; they had been so down with the state of the game that they hadn’t even said happy Christmas as they’d last left. So, they came to the stadium on the night before Christmas eve, almost a hundred of them, and they sung and they had a drink, and they remembered that supporting a football club is about your neighbours on the terrace as much as it was ever about your position in the table.

They came back the next year, and a few more joined. And then the year after that, too. And Union’s Weihnachtssingen went on and on, inexorably growing and puffing out its chest to the point where tens of thousands were coming along every year and this peculiar Christmas celebration had stretched out of all possible scale to its humble origins.

The sound of church bells rings out around the stadium and the floodlights' lights are suddenly killed, until all that remains is the flickering of thousands of candles in the darkness and the beginning of Nina Hagen’s own hymn to the club, summing up this curious mixture of football and festivity - “Osten und Westen, Unser Berlin,” it goes.

Then the voices of almost 28,500 people – both Unioner and not – swell as one with those of the local Emmy-Noether-Gymnasium school choir, segueing almost seamlessly into the traditional opener, “Guten Abend, Schön Abend”.

In the pauses they chant “Eisern… Union”, a call and response, as is done between the Waldseite and Gegengerade on matchdays, with a vigour that belies the fireside atmosphere, before moving into “Oh Tannenbaum” without skipping so much as a beat.

Of course they sing that one, just as they have ever since that first ever time twenty-odd years ago. There’s a story, recounted in Kit Holden’s book, “Scheisse! We’re going up”, that conjures up the irreverent nature of that first evening. They’d reached the line about “Wie grün sind deine Blätter”, when a voice piped up.

“Leaves? Leaves? Christmas trees have got needles, not leaves”.

There is often talk of how football now resembles the closest we have, in largely secular lands, to the communality of a church. In the absence of an all-powerful God, we still need something to pull us together at times. This is where that call and response comes in, the bells ringing out, and even Reverend Ulrich Kastner reading the story of Christmas. Eisenbeiser described the Weihnachtssingen in that book as “the biggest church service in Germany not held in a church.”

But you don’t need to believe in God, just your neighbours, no matter where they come from nor what they believe. Santa will also appear, to give a reflection on the footballing year, if done in verse with a hefty dose of scurrilous humour thrown in. Two years ago, he said the immortal lines, in English, “Football is for you and me, not for fucking industry,” with glee. Ho! Ho! Ho!

There are the tones of the “Eisernen Bläser” brass band and the Mahlsdorf men’s chorus, and that beautiful moment when stadium announcer Christian Arbeit, on trumpet, is joined by his mum and dad, on clarinet and trombone respectively, as the three enter into a gorgeous, fragile, “Oh come all ye faithful”.

Because what was this about if it wasn’t about family, after all?

Arbeit had inherited the natural qualities (“big lips and long arms”, he says with a smile) of a trombonist from his dad, a mainstay of the DDR’s swing band scene, who had met Louis Armstrong in 1965 when he came to East Berlin, and has never stopped playing to this day. But an Unioner, to the core, his contrary streak ran a mile wide. He chose to play the trumpet instead of his birthright.

So, after he had taken over the gig as master of ceremonies, and a musical accompaniment was needed for the show, there was only one clear choice as to who should form the band. The three of them will take to the stage again on Tuesday. The kindred heart of the warmest show in town.

Many have asked if they, too, could use the Weihnachtssingen as a chance to extend their reach, to flog their latest record, to shoehorn themselves into an event that is now copied across the land – even if few will ever manage to achieve the closeness and spirit of the original. They were always told, no. This one belongs in the family.

It matters little that it often rains, or that it invariably starts later than planned, because every year when the Unioner hug and say “Happy Christmas” at the end, and as they make their ways home through the forest unconsciously re-enacting the trip of TeeCee and Torsten and all their friends all those years ago, they remember what is important, in football as in life.

This years Weinachtssingen will be broadcast live on AFTV. Watch the complete film from last year here