Wish it could be Christmas every day? Within half an hour, this mammoth space - all 600,000sq ft of it: 7.5 football fields in total - is a pullulating mass of people. As the Christmas rush takes hold, 600,000 people a week - more than the population of Cornwall - pass through. Many will emerge on to Oxford Street with at least one of the store's four million products tucked away in the famous Pantone 109 yellow bags. Like many Londoners with a penchant for posh trainers, I've been here hundreds of times, but never have I stood here so still and for so long just watching people. Looking down from the fourth-floor atrium, the scene brings to mind a termite colony invading a neighbouring tree stump. Only here, the driving impulse is not survival but designer handbags. To Geraldine James, the unending tide is an affirmation. "I defy anyone not to find a gift in here somewhere," she says, regarding the fourth-floor Christmas gift shop with the look of someone whose plan is coming perfectly together. James lives on Christmas cheer, but then she has to. For the past nine years, she has been the store's Christmas buyer. For her, the season never ends. There isn't one month when I am not thinking and planning and buying for Christmas. Five years ago, I would have at least had some brain space in April. Not any more," she says with a smile. James, one can't help feeling, is the natural heir to Harry Gordon Selfridge, the shop's founder. For, as January's ITV mini-series Mr Selfridge showed, the idea of the Christmas shopping experience practically began with the store's opening in 1909. Selfridge was the great showman of shopping; a retail genius. He believed that shopping was not just a means to an end, but an activity in itself - and we lapped it up. He turned his vast neo-classical building on Oxford Street into a tourist attraction, putting on fashion shows on the roof, buying Blériot's Channel-crossing plane for the atrium and pioneering steel-frame-supported plate-glass windows, which could be enticingly dressed with products from the racks, shelves and cabinets within. He would utilise any gimmick in search of a sale, employing circus performers to entertain queuing visitors and pumping the aroma of chestnuts and cinnamon across the six heaving floors at Christmas time. James wouldn't countenance anything so heavy-hoofed and hucksterish today. But she clearly does have that Selfridgian ability to divine what the customer wants and then sell it to them at mark-up (the store made £133m in profit last year, a fair chunk of it at Christmas). When I visited on the second Tuesday in December, it was the first day of Christmas 2014 for her. Later that afternoon, she attended a seasonal "wash-up," at which her team and other heads of departments met to discuss what went well and what went less well this Christmas, despite there being two weeks of the season left to run. The New Year, though, is when the real heavy lifting begins. "On the 11th of January, I fly to Atlanta for their vast Christmas trade sale," James says. Then I go on to France, pop back for the Christmas show in Harrogate, and then fly to the Czech Republic to buy my stocking fillers. And after that, it's the trade show in Birmingham," she says. It can get tiring, of course, but at least I've bought all the gifts I need for the Christmas shop by mid-March. This year's haul - which goes into the Manchester and Birmingham stores, too - includes pet gifts, neon and dip-dyed decorations and 120 different types of advent calendar, all of which have been on sale since - whisper it - 5 August. Is that not a bit early, I ask? At that time of year, people are still buying swimming shorts. Surely people don't want tinsel? "You'd think that, but I linger on the fourth floor when we open the Christmas shop and listen to people and I often hear "Oh, my god, in August!" and those same people then leave with a basket full of baubles," she says. Since Galen Weston, the Canadian retail billionaire, bought the then-ailing Selfridges in 2004 for £598m, the store has experienced a renaissance. For the past three years, it has been crowned "Best Department Store in the World" at the Global Department Store Summit and last year had sales of more than £1bn. The success has not only been commercial, though, but creative, too. The store is renowned for its bleeding-edge look and high-design ideals. The Weston family has turned the shop into a sensory playground, covering the hard commercial reality; the sell, sell, sell, with some of the most head-turning installations and decorations on the high street. The person planning it all? Linda Hewson, head of the creative department. Hewson, shrewd and calm and not at all the Ab-Fab type that I expect, is the general commanding Operation Christmas. Each year, her job is to reinvent it anew. Thirteen months ahead of the season, she and her team must come up with the "Christmas concept" which will inform everything from what the departmental buyers purchase, to what the decorative theme will be in store, what music will be played and what will fill the 27 windows. Her department is comprised of the ultimate trend forecasters. My research team goes to art galleries, exhibitions and the theatre to try to divine future trends in art. They then chat to the buyers and come up with the concept," she says. This year that was "Destination Christmas," which meant, in essence, that they turned it into a giant gift shop, complete with mini pop-up shops scattered all over the store and the fourth floor turned into Christmas Village. It takes six months of planning and then 4,000 staff hours to create a visual language that says: "Come all and shop 'til you drop." Success for Hewson comes in two guises, she explains. We want the place to be magical and creative, but also a showcase for what we sell. I think this year, more than ever before, we have managed to pull that off - especially with the windows. Although she may be proud of what her team achieves with the Oxford Street windows - this year they plumped for giant gifts in a snowy wonderland - logistical challenges lie behind every pane for the dressers. Some of the windows can be accessed only by tiny doors. Others must be entered through interconnecting passages from other windows, and still more have to be reached through concessions. You find yourself constantly apologising to the staff at the Fendi concession for dropping glitter all over their stock. And then, after three days of the glass being covered, you unveil them and wait for the postcards and letters to roll in," she says. Everyone has a view on what Selfridges should be like. As much as they take pride in the décor of the store - its visual language - there is another lodestar in Selfridges' sky: food. The food hall is presided over, without fear or favour, by Nicola Waller. Her role is manifold, but above all things, she must ensure that her particular part of the shop does not run out of the stock at the height of the Christmas rush. Woe betide the store that runs out of Christmas pudding. "If people come in on the 23rd, our busiest day, and try and buy a Christmas pudding, and you've run out, you are going to have some angry customers," she says, before adding: "That, and a snow storm in the week before, are the terrors that keep me awake at night." Her metric of success is simple enough to understand. She wants her section of the store to "surprise, amaze and amuse." Who is her target audience? "Everyone from oligarchs to students," she says, not missing a beat. In this case, that translates as selling giant Hershey bars (£39.99 a kilo) in one corner and £3,050 Beluga caviar in another. She explains that, perhaps more than anywhere else in the shop, her department needs to stay half a step ahead at Christmas. It is a delicate balance. Less than that and you effectively fall behind; more and no one knows what the hell is going on," she says. Maeve Wall is perhaps the yin to Linda Waller's yang. She is there not to excite our senses and prick our interest, but to calm things down. She describes her job, store director, as being a fire fighter. Impeccably dressed and unflappable, she glides across the marble floor spreading calm as she goes. My job is to make it easy for everyone: to take the stress from shopping. At Christmas, that means employing and training 2,000 temporary staff (the store's permanent employees number 1,500), ensuring seven 40ft articulated lorries full of stock make it down Oxford Street every night and generally making certain that everything is ticking along nicely. To do this, she works 60-70 hours a week in the festive season and walks 10 to 12 miles a day. You have to empathise with people. Spot the stress spots and turn that around," she says. As if to prove this, a fire alarm is activated as we are talking. Immediately her BlackBerry goes to her ear, her hand forms a barrier across her mouth. After a few minutes, the phone comes back down. "A false alarm," she says in Head Girl tones. Now, let me show you Mark's bar. She is the Halls Soother of Selfridges. As I make my way past the pink glittered staircases and politely refuse a store programme from one of the Christmas hosts, it occurs to me that at this time of year, more than ever, this place becomes a city state; a fully-staffed principality, employing thousands of minds, all dedicated to prefiguring our desires, distilling them and turning them into products we can walk out into the world with, flushed with the thrill of consumption. Here, before me, is the ultimate exercise in the commercialisation of Christmas - and, by the looks of the crowds, we wouldn't want it any other way.