The Big Question: What's behind Christmas traditions – and just how traditional are they?

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Why are we asking this now?

Tonight you will put up the stockings, Santa Claus will arrive on his sleigh drawn by reindeer to slide down your chimney in his trademark red suit with a sack over his shoulder, and tomorrow you will open presents under your decorated Christmas tree, eat turkey and mince pies, and promise yourself that next year you won't leave it until the last weekend to write your Christmas cards, because it is Christmas, and it is traditional. But do you know how old these "traditions" actually are? Some are ancient, some are newer than you think.

Why is Christmas Day on 25 December?

The Bible offers no date for the birth of Jesus, which probably was not in the year 1AD, but a few years earlier, and may or may not have been in December. The celebration of the birth of Christ on 25 December dates back to the fifth century, when Constantine made Christianity the official religion of the Roman Empire.

The date was chosen to coincide with the winter solstice and the Roman festivals associated with the shortest day of the year, which falls between 22 December and 25 December. This was seen as the day when the Romans celebrated Dies Natalis Solis Invicti – "the birthday of the unconquered sun". It was also Jupiter's birthday and, further back, the birthday of his Greek equivalent, Zeus. In Eastern Europe, the various Orthodox churches – the Russian, Greek, Armenian, Serbian et al, follow the old Gregorian calendar, and in which Christmas Day is 7 January There is no Santa Claus in the Gospels.

Where did he come from?

Nearly 1,700 years ago there was a bishop of Myra, in Asia Minor, who was imprisoned under the last pagan Roman Emperor, Diocletian, but reinstated under Constantine. A cult grew up around him in Greece and spread outwards, and he became the patron saint of children, among others. An old legend about him is that there was a poor man who could not afford dowries for his three daughters, until bags of gold were tossed through an open window by St Nicholas, landing in the stockings drying in front of the fire. In Holland and Germany, there was a custom that St Nicholas was the secret bringer of presents for children on 6 December, his feast day.

When did he start sliding down chimneys?

After the American revolution, New Yorkers tried to rediscover their Dutch roots, and revived the feast of St Nicholas, and his legend. The writer Washington Irving took the mickey out of this revived cult in a satire published in 1809, called Knickerbocker's History of New York. In it, St Nicholas appears as a fat, jolly figure, dressed in fur, with a clay pipe and beard, who slides down chimneys.

And the reindeer?

On 23 December 1823, the Troy Sentinel, in New York State, published an anonymous 56-line poem variously known as "A Visit from St Nicholas" or "The Night Before Christmas." which fused the feast of St Nicholas with Christmas, and had the St Nicholas that Irving created arrive on Christmas Eve in a sleigh pulled by eight tiny reindeer. The author was probably a Professor of Oriental and Greek Literature named Clement Clarke Moore, who did not want to sully his fine academic reputation by putting his name to some nonsense he wrote to amuse his children. The reindeer had names, but none was called Rudolf. He of the Red Nose was created by an advertising copy writer in 1939.

And when did Santa get that ridiculous red outfit?

In 1863, the cartoonist Thomas Nast began a series of drawings in Harper's Weekly, based on "The Night Before Christmas", in which Santa Claus, as he had now become known, could be seen with flowing beard and fur garments. Around 1869, he turned up for the first time in a bright red suit, with a white belt, but he was not invariably dressed in red until the mighty Coca Cola corporation appropriated him for an advertising campaign that began in 1931, and ran every Christmas for 35 years. That is also when the reindeer became full size. In Britain, this American import merged with an older folk hero called Old Christmas, or Old Father Christmas, a fun-loving heavy drinker who seems to have arisen in reaction to the Puritans.

So where did the idea of a Christmas tree come from?

As you are sweeping up those annoying pine needles next month, you can blame the Germans. When Victoria married Prince Albert, he brought over the German habit of decorating a tree, which the English adopted out of reverence for their Queen. What started the Germans off is lost in the mists of time, but one story is that in 722AD, St Boniface came upon pagans who were about to sacrifice a child at the base of a huge oak. To save the child, he cut down the oak, and when a fir tree grew up at its base, he declared that this was the tree of the Christ child.

Why do people eat turkey?

Long ago, it was the smell of roast goose or the head of a boar that filled the Christmas air in Britain. Then in 1526, a trader named William Strickland imported six turkeys from the US and sold them in Bristol, for tuppence each. The birds were popular because they were tasty, and practical. Cows were more useful alive, chicken was more expensive than it is now, and other meats were not as popular.

And mince pies?

Mince pies are the modern descendant of the Christmas Pye, a large dish filled with shredded pigeon, hare, pheasant, rabbit, ox, lamb, or mutton, mixed with fruits and sugar. It had an oblong shape, said to resemble Jesus's cradle. After 1660, they became more like the pies we eat now.

What about Christmas cards?

The first person ever to think of selling Christmas cards was a civil servant named Henry Cole, who had worked on the introduction of the first postage stamp, the Penny Black, in 1840. He was too busy that year to write to all his friends, so he commissioned a designer named John C. Horsley, of Torquay, to design a card with the words "A Merry Christmas and a Happy New Year To You". In 1843, the year that Charles Dickens wrote A Christmas Carol, Cole went a step further, by commissioning 1,000 cards. He used some, and put an advertisement in the press offering the others for sale at 6d each. One card from that batch was sold in December 2005 for £8,500.

Has the Christmas holiday always lasted this long?

As a religious festival, Christmas used to be almost as long as the festival of consumer extravagance that it now is, but it started and ended later. In the 1840, a Roman Catholic priest named Dom Geuranger published what many still regard as the definitive study of the liturgical year, which identified Christmas as beginning on 25 December, and ending with the Purification of the Blessed Virgin, on 2 February. "The custom of keeping this holy and glorious period of forty days as one continued Festival has every appearance of being a very ancient one," he wrote. So all those shopping binges, office parties and hangovers that preceded Christmas are one vast misunderstanding. Christmas begins tomorrow, and you should carry on celebrating it right through January. Get to it.

So should I believe in Father Christmas?

Yes...

* If he doesn't exist, why did parents riot when a pilot recently refuse to fly them to Lapland?

* What about the letter to S. Claus from Inland Revenue, querying his claim for travelling expenses?

* There are 300,000 living organisms yet to be classified – couldn't one be a reindeer that can fly?

No...

* Santa would need to visit 896 homes a second to fill all the stockings

* Only Kathy Staff, the actress who played Nora Batty, really knew how to fill a stocking, and she has just died

* When Chico Marx was told to sign a clause that would prove he was sane, he rightly said: "You don't fool me: there is no Sanity Clause"