BBC iWonder - Why do we still love Christmas number ones?
BBC iWonder
1. The Xmas factor
Music journalistNicola Meighan
Waiting with bated breath to discover the Christmas number one is as much a British festive tradition as the Queen's Speech, bad jumpers and over-cooked turkey.
Even in the era of digital downloads, the Christmas edition of Top of the Pops remains appointment viewing.  With music being freely available, legally or illegally, what prompts us to still put our hands in our collective pockets and push one song rather than another to the top spot?  Why do bands still record festive songs determined to have the best-selling song at this time? Why is the Christmas number one held so close to our hearts?
2. The festive chart battle begins
Slade's Noddy Holder and Wizzard's Roy Wood tell the story behind the epic battle for the 1973 Christmas number one spot.
For the first bespoke Christmas number one, we have to travel back to Dickie Valentine's Christmas Alphabet in 1955. The rest of that decade's Christmases were also dominated by classic crooners.
1960s yuletides were ruled by The Beatles, who bagged four festive best-sellers.  They remain the most successful act in the history of the Christmas charts.
The modern fascination with the Christmas number one began in the 1970s. 1973 found the UK facing mass industrial unrest, IRA bombings, the threat of petrol rationing and, in an attempt to save on power, a three-day working week.
At that time, glam rock was a huge music phenomenon with performers dressed in garish, over the top outfits.
Slade and Wizzard, two West Midlands bands, were the biggest performers in this genre.
At a time when people were looking for a bit of escapism, their singles Merry Xmas Everybody and I Wish it Could Be Christmas Everyday fused the fun of the season with the outrageousness of glam rock to create the perfect Christmas record.
The public bought into the excitement of seeing which record would come out top on Christmas Day as both sets of fans clamoured for their idols to be seen as the best.
Slade triumphed with Merry Xmas Everybody staying at number one until well into 1974.  Both songs remain staples of Christmas party playlists.
That lasting appeal is borne out by cold, hard cash. Slade's Christmas perennial has sold over a million copies in the UK and racks up another 100,000 in downloads every year.
In a 2007 poll, Merry Xmas Everybody was voted the most popular Christmas song ever. It has also topped a list of the highest-earning festive tracks.
Digital Spy: Classic Christmas chart battles
3. The festive musical formula
Even some of our favourite Christmas songs failed to make it to the cherished top spot. Enjoy a selection of brilliant near-misses.
Not only can a Christmas number one secure your place in the history of popular music forever, it can also be a huge seasonal money earner, providing a guaranteed income in an industry where trends can change rapidly.
Many bands from Wham to the Pogues to the Spice Girls have sung a Christmas single although not all made it to number one.
Is there such a thing as a winning formula to secure that Christmas chart-topper? Successful songs have been known to draw on stock tricks to make us feel festive.
A seasonal title helps. There have been six yuletide chart-toppers with the word Christmas in the title – seven, if you count Slade's Merry Xmas Everybody.
Ballads are a good bet. Perhaps emotionally and socially they evoke memories of the last dance at Christmas school discos.
Psychologists say we are more likely to seek solace in sadder music during times of economic uncertainty. 1974 saw a further year of IRA bombing, industrial unrest and two general elections. Mud crooned to the top spot with Lonely This Christmas.
Many of our best-loved hits tap into our memories and nostalgia. Some are explicitly Christmas-orientated, such as Johnny Mathis' When A Child Is Born from 1976.
But some songs cleverly conjure Christmas by inference- a sleigh bell here; a snowy video there - like East 17's Stay Another Day in 1994.
Charity singles, novelty songs, one-hit wonders, bizarre collaborations, the search for the elusive Christmas number one magic formula has seen them all.
Official Charts Company: Every Xmas Number 1
4. The novelty song
There’s No One Quite Like Grandma by St Winifred’s School Choir topped the chart in 1980. With his murder occurring in early December, John Lennon's final single (Just Like) Starting Over had been seen as that year’s sure-fire hit.
In 1968 a comedy ditty by The Scaffold called Lily The Pink topped the festive charts and became the first-ever novelty Christmas number one.
Every few years since, we have taken leave of our senses and bought the most mind-melding records en masse.
There are a few arguments as to why novelty Christmas singles are so popular in the UK.
One reason they are popular is for a bit of escapism.   We like to seek a bit of fun at the end of the year and distract ourselves from tough economic or political problems.
That certainly chimes with Lily The Pink, whose surrealism was at odds with 1968's social unrest and CND marches.
Another theory is that people sometimes only buy records at Christmas and like to get caught up in the buzz.
This argument might explain the success of Bob The Builder's Can We Fix It? in 2000 and the St Winifred's School Choir with There's No One Quite Like Grandma in 1980.
The St Winifred's School Choir 29 years later
5. The charity single
Midge Ure explains how charity supergroup Band Aid was formed following news reports from the 1984 famine in Ethiopia.
1984 ushered in the festive single with a social conscience.
Do They Know It's Christmas? by Band Aid remains the biggest-selling Christmas number one of all time.
The stark message about poverty and starvation in Ethiopia tapped into our collective guilt, while the ceaseless campaigning of Bob Geldof - bolstered by an A-list musical cast - attained unprecedented media coverage and a place in British popular history.
One million records were sold in the first week of release. It has since sold almost four million copies in the UK and raised £8m. It was re-released in 1989, 2004 and 2014, topping the chart each time.
The original kept Wham!’s Last Christmas off the top of the charts, proving that Christmas songs still had the ability to tug on our social conscience and make us think of those less fortunate at this time of year.
Band Aid was hardly a one-off, either. The Justice Collective's Hillsborough fundraiser He Ain't Heavy, He's My Brother topped the charts in 2012 with the Peace Collective’s World War One inspired All Together Now making its bid in 2014.
If the ultimate Christmas message is to help others less fortunate than ourselves, then perhaps the charity single is the ultimate festive pop medium.
Pictures and quotes: Band Aid 30
6. The television factor
Following the formation of Girls Aloud on Popstars: the Rivals, the girls reached the Christmas number one with Sound of the Underground.  Clip: Christmas Top of the Pops 2002.
By the 1990s, the focus and hype around the Christmas number one had become big business. For the past four years, over a million singles have been downloaded on Christmas Day alone.
These statistics have not been lost on TV executives and record labels. Since the early 2000s talent shows have exploited the vast sales potential of the Christmas chart.
ITV's 2002 series Popstars: the Rivals created two groups - Girls Aloud and One True Voice - who competed against each other with the sole view of achieving the Christmas number one that year. The honours eventually went to Girls Aloud.
Beginning with Shayne Ward in 2005, the reality-pop leviathan X Factor threatened to hijack the Christmas top spot forever with a run of six Christmas number ones in nine years.
The fear began to grow that the X Factor had a monopoly - that the excitement of pledging allegiance to a record for Christmas number one had gone.
But the charts belong to the public who were tiring of the manipulation of the chart by business interests.  By 2009 a rebellion against X Factor was afoot, driven by social media.
As a result, that year's X Factor winner Joe McElderry, singing The Climb, lost out to one of the most non-Christmas songs in music history - hardcore rockers Rage Against The Machine and Killing In The Name Of.
For the public, the annual race to the top offers an chance to buy into a communal sense of romance, nostalgia, charity or even rebellion.
In 2014, as Band Aid 30 prepare to take on the X Factor winner, the growing speculation and sense of excitement surrounding the eventual Christmas number one continues to highlight how important the Christmas Day top tune is to us all.