Today, when Parkhead hosts a #19 Legends Match game featuring a clutch of old heroes and a smattering of showbiz stars, that attendance will be smashed with a sell-out.
More than a club? The very essence of what makes Celtic special will be demonstrated by the capacity crowd.
The game between a Stiliyan Petrov XI and a Celtic XI has even caught the imagination of BT Sport, who will screen the event live.
Petrov's foundation - created to help others who, like the 34-year-old Bulgarian, have suffered the devastating effects of acute leukemia - Celtic Charity, and the Trussell Trust - which runs food banks in Glasgow and across the country - will all benefit hugely from the proceeds of the day.
But, the fact Petrov has recovered well enough - he has been in remission for the past year - to be there to lead out his side against so many of his ex-Celtic team-mates is what will raise the biggest cheer.
And Tom Boyd, who captained the midfielder when he arrived at the club as a homesick 20-year-old and who will be pulling the boots back on as part of the 'home' side, believes Petrov is an inspiration to everyone.
"It's when you see the magnificent response to events like this that you understand what people mean when they say Celtic is so much more than a football club," said the 47-year-old with pride.
"Recently, I have been privileged to be involved in a number of special occasions, including the 125th anniversary of the club, and the fans have done us proud every time.
"But, this one really is something else, and the story of Stiliyan's recovery from leukemia really is inspirational.
"He is such a popular guy, and for this to happen to him really was a shock to us all.
"I can honestly say I don't know anyone who has a bad word to say about Stiliyan. He's a great professional and a wonderful family man
"It's typical of Stiliyan that the first thing he does when he is getting better is think of others and get involved in this game.
"Which is why everyone wants to take part in it."
It certainly has drawn a stellar cast, which includes One Direction's Louis Tomlinson, film and TV stars Stevie Graham and Martin Compston, and most of the players who played alongside Petrov as he picked up a clutch of medals during seven years at Parkhead.
Current manager Neil Lennon is one of those who has agreed to take part, though he will have to get used to taking orders once again from his mentor, Martin O'Neill, who is in charge of the Celtic XI.
With Kenny Dalglish - who, along with John Barnes, was responsible for bringing Petrov to Parkhead in 1999 - calling the shots from the other dugout, and trying to get the best out of men like John Terry, Dimitar Berbatov, Roy Keane, Shay Given, Gareth Barry and former Westlife star, Nicky Byrne, it promises to be a feast of football.
Boyd is concerned there is a heavy age advantage for the opposition squad, with so many players still involved at the top level.
But he accepts it will all add to the spectacle of the day, and is delighted the real winners will be the nominated charities.
"Alan Stubbs and John Hartson have had their health problems in the past, and, like Stiliyan, they recovered and went out and raised awareness of the issues involved," said Boyd.
"They can use the fact they are recognised as personalities to highlight just how much work is being done on a daily basis by so many others behind the scenes.
"As always, Celtic have added all their weight to this, and the fans have got behind it, too.
"I just hope they are not coming along expecting to see silky football from us.
"We'll be doing our best, but I think it could be a bit one-sided when I look at how many younger guys are in the other squad."
A decade after he hung up his boots, Boyd retains a high level of fitness and enthusiasm, but admitted: "For our group, it's really about nostalgia and having a good time together again because we really don't get the chance to meet up all that often.
"That said, I still do a bit of work in the lounges at Celtic Park on match days, and I'm sure I'll take some stick if my performance is not very good.
"You can bet there will be a fair bit of slagging going on between the players, and that Martin O'Neill will be reminding us that Celtic don't play friendlies.
"I looking forward to hearing some of his inspira-tional words again before tomorrow's game."
Perhaps O'Neill might be well served to keep some in reserve for when his troops start to tire, as a number of those involved are packing a bit more timber than when they last starred in the Hoops.
"Lenny looks as though he has enjoyed the fruits of the Champions League," said Boyd with a grin.
"But it's great he's going to be pulling the boots on again, and I am sure he will nullify the opposition, just as he always did.
"It will be interesting to see how Paul Lambert plays lining up against Lenny and the rest of his old team-mates. Maybe he should be playing a half for each side."
Should Boyd be feeling the going tough today, he already knows where he is going to draw inspiration to keep going.
Next weekend he will undertake a 64-mile static cycle run for charity. And, joining him in the gym-based challenge will be life-long Hoops fan, Geraldine Byrne, who is awaiting a lung transplant.
Like Petrov, 46-year-old Geraldine is facing up to her health issues with guts and determination, and Boyd has nothing but praise for this exceptional lady.
"All we are being asked to do is play football against lads who are bit younger than us," said Boyd. "What Geraldine is doing is amazing and really inspirational."