Just a few years back, the switching on of London's Oxford Street Christmas lights was the big event that heralded the holiday season in the U.K. These days Christmas begins with the arrival of a spectacular holiday commercial from department store John Lewis.

Ahead of the campaign's debut, rumors of singer Lily Allen's involvement abounded in the national media. Meanwhile, rival Marks & Spencer jumped in early with the (online) launch of its own campaign, a big-budget extravaganza starring Helena Bonham-Carter as the Wizard of Oz and model Rosie Huntington-Whiteley as Alice (in Wonderland) crossed with Dorothy.

Marks & Spencer's Wonderland-meets-Oz extravaganza (2013)

The John Lewis spot -- a Disney-style animated tale of a bear that has never seen Christmas -- was trailed with teasers on social media, then finally projected onto London's South Bank before it took over an entire two minute break during "The X-Factor" (a decision that had to be signed off by Simon Cowell himself). John Lewis is further capitalizing on the spot this year with a number of product tie-ins selling at the retailer.

John Lewis's animal who had never seen Christmas (2013)

"It has become like our version of the Super Bowl -- and I don't think anyone even saw that coming," said Ben Priest, executive creative director of Adam & Eve/DDB, which created the John Lewis ad.

Mick Mahoney, executive creative director at Marks & Spencer agency Rainey Kelly Campbell Roalfe/Y&R, described the phenomenon as an "arms race." He said it began with John Lewis "upping the ante" two years ago with its campaign "The Long Wait":

John Lewis' "The Long Wait" (2011)

But this year has been "particularly extraordinary," according to Mr. Mahoney. "People seem just desperate to talk about it," he said.

So why the seasonal hysteria? Social media has undoubtedly fueled the fire. But Ben Fennell, CEO at BBH, also put it partly down to the success of "The X-Factor," which now regularly draws audiences of eight million and up. Beginning with BBH's campaign for Yeo Valley in 2011, brands have taken to booking entire ad breaks and look set to pay up to 200,000 pounds for a spot during the final -- which is shown just before Christmas.

This year, however, BBH has created a low key campaign for grocery chain Waitrose, focusing on its charitable donations. "We predicted the other retailers would do big extravaganzas and we loved the contrast of Waitrose telling a simple story, Mr. Fennell said. "We're still living in a tough economy and these blockbusters feel a bit disconnected to shoppers."

Sainsbury's has similarly chosen to focus on ordinary people, with its Christmas effort involving clips from a 50-minute YouTube documentary from director Kevin Macdonald, a riff on his much-buzzed about crowdsourced Google film with Ridley Scott, "Life in a Day." The project, which will premiere at BAFTA, was 14 months in the making.

Sainsbury's "Christmas in a Day" (2013)

Marks & Spencer, however, believes Christmas is still a time for fantasy. "It's a time of year when the normal difficulties are suspended," said Rainey Kelly's Mr. Mahoney.

Adam & Eve's Mr. Priest said this year's John Lewis effort has evolved out of the public's love for previous campaigns -- and admits he breathed a sigh of relief at the ad's mainly positive reception. But now that its ads involve product spin offs, e-book tie-ins and apps, the seasonal planning process is beginning ever earlier in the year. "We've barely had to time to digest the turkey before we're on to the next one."

Even though the U.K. is only just emerging from recession, Christmas ad budgets are growing: the grocers alone are estimated to be spending up to 9% more on ads this year than last. Will that translate into sales? For some, the proof is in the (plum) pudding: the last three Christmases for John Lewis have been its most successful.

Tesco's Christmas through the years (2013)

Marks & Spencer clothing sales have fallen recently, but Mr. Mahoney said the campaign is also about branding and attracting a younger audience. Ray Shaughnessy, co-creative director on Tesco at W+K London, says its ad -- a nostalgic spot depicting a family celebrating Christmas over the years -- aims to create "emotional resonance" with the brand that shows that it understands Christmas.

Nevertheless, the pressure on retailers and their agencies to create something spectacular has never been fiercer. "Next year, everyone will be looking at changing it up," Ms. Shaughnessy said.