Pre-Christian Roman Traditions:
Much of what people associate with Christmas, a holiday that is supposed to be about the birth of Jesus, actually pre-dates Christianity. Traditional Christianity celebrates the death of holy people, not their birth. In 274 CE, though, pagan emperor Aurelian proclaimed Decemer 25th Natalis Solis Invicti, the festival of the birth of the invincible sun. Saturnalia already occurred around this time along with many other celebrations. Christians took over this and other Roman festivals.
Pre-Christian German Traditions:
Germans of the north also held mid-winter festivals. Evergreen trees and holly were important because they held their green colors despite the harshest winters. Another Nordic tradition is the Yule Log the origin of the word yule is disputed, but it may be related to the god Yolnir, Jol in Old Norse. In his name people celebrated a 12-day festival of eating, drinking, and merriment. One legend about Yolnir has him hanging himself on a tree and piercing himself with a spear.
The most important Christian aspect of modern Christmas is Santa Claus. This character is traced to Saint Nicholas, a patron saint of children on whose day (December 6th) gifts were given to kids. Beyond the idea that that people are supposed celebrate the birth of Jesus on this day, this is all that Christianity has directly contributed. Many Christians acknowledge this and some denominations dont celebrate Christmas at all, regarding it as little more than a dressed-up pagan holiday.
Washington Irving and Santa Claus:
Santa Claus may have Christian origins, but modern notions of Santa Claus can be traced to the writings of Washington Irving writings often meant as satire, yet which also often ended up being read as factual. His stories about New Amsterdam being dominated by a cult which celebrated the figure of Saint Nick were popular and he often returned to the figure of Saint Nicholas, even writing about old fashioned Christmas celebrations which he simply made up.
Charles Dickens and Christmas Feasts:
Charles Dickens story A Christmas Carol has not been his only influence. He seemed obsessive about the holiday, and more than once he depicted lavish Christmas feasts rife with old fashioned traditions which were little more than literary creations. Nevertheless, people followed right along, enchanted with the atmosphere he created and wishing to capture it for themselves. As with Washington Irving, people accepted fiction as if it were true and thus allowed life to imitate art.
Queen Victoria, German Christmas, and Christmas Trees:
Queen Victoria came from a German family and unlike English families, she celebrated Christmas with a small decorated indoor tree. Then, as today, people obsessed over royalty and they were particularly infatuated when their teenaged Queen ascended the throne. Then, as today, people wanted to be fashionable and adopt what their role models did in this case, the tree tradition. It wasnt quite the Christmas tree we know today, but it was romanticized and took on a life of its own.
Clement Moore, Christmas Eve, and a Secular Santa Claus:
Clement Moore is credited with writing the poem A Visit From St. Nicholas which almost singlehandedly created the modern American conception of Santa Claus, what he looks like, and what he does (without ever using the name Santa Claus). At this point Santa Claus is fully secularized. There is no reference to sainthood, Christianity, or Jesus. Instead, Santa has a pagan image he is, after all, described as an elf. Nast also establishes Christmas ideas like hanging stockings by the fireplace.
Thomas Nast and Santa Claus:
Illustrator Thomas Nast is the one who gave America the physical pictures of Santa Claus, thus providing a common, shared vision of the primary symbol of an increasingly secular holiday. Nast also added numerous key details about his life which hadnt been used by others before: Nasts pictures showed Santa reading childrens letters, watching their behavior, writing the naughty and nice lists, and living at the North Pole.
Francis Church and the Meaning of Christmas:
An editorial writer of the New York Sun, Francis Church wrote the famous response to Virginia OHanlon about Santa Claus. As Tom Flynn describes, Church united strands of Christian mysticism, nineteenth-century Transcendentalism and Romanticism, and general distrust of scientific skepticism. Church described a coherent vision of what Christmas was supposed to be, replacing a literal Santa that children believe in with a metaphorical Santa representing a spirit of generosity and love.
Christmas Shopping and Commercialization:
It was just after World War II that people complained about over-commercialization of Christmas and started to try to put Christ back into Christmas. But why at this point in history? Blame it on the war: during the conflict, people had to shop early in order to get Christmas gifts to the troops overseas in time. Merchants, of course, benefited from the early shopping and made a point of reminding people to shop early even after the war ended.
Thus the lengthy holiday shopping season was born and its been getting longer ever since. Over time people have learned to try to shop even earlier in order to beat the maddening holiday rush and retailers, being the good public servants that they are, have sought to accommodate them with earlier and earlier holiday sales. By and large, the spirit of giving has become predicated upon a spirit of buying.
Such is our modern Christmas: a large number of ancient pagan practices, a few pieces of Christian traditions, and a large number of modern creations which are almost entirely secular in nature, no matter where they got their inspiration from. I see little room and little need for any Christ in all of this - but more importantly, I see little place where a Christ could be put back into the mix.
So when you find someone talking about putting Christ back into Christmas, you can ask them what part Christ really played in Christmas to begin with. Although you may not be interested in celebrating holidays with any religious trappings whatsoever, Christianitys hold on Christmas, from a religious perspective, is rather tenuous. If youre an atheist who would like to enjoy the holiday, you should be able to do so without giving Christianity a second thought.
See Tom Flynn's The Trouble with Chrismas for more on this.