Why should we take down our Christmas decorations on Twelfth Night (January 5th)? According to this website it's because the wood spirits will bring us bad luck otherwise.
Christmas is the second most important festival in the Christian calendar, and it has become the biggest festival in modern Western society. But Christianity plays a diminishing role. In the many-layered concoction that Christmas has become, Christianity has left a number of distinctive strata. The carol service, the children's nativity play, concerts of sacred music and midnight mass. These come on top of European pagan strata, and underneath more recent secular ones (Father Christmas and all that). And as this website shows, more secular types sometimes try to escape the Christian nature of Christmas by appealing to the more ancient traditions.
By and large Christians seem to be accepting this retreat with good grace. We did get a grump from the Pope about excessive commercialisation, but that was about the only grumbling I picked up this year. The Archbishop of Canterbury was on the front foot and positive. This is wise. The trouble is that the Christian Christmas message has diminishing evangelical weight.
Christian Christmas (it's a stark sign of the retreat that I have to call it that given that Christianity is embedded into the festival's very name) is about the fact of Jesus's incarnation. It's about who he is, rather than what he said. And this takes us into the areas of Christian doctrine that resonate least with the modern world. "God sent his only Son to save the world" is an expression of faith has become a meaningless string of words to people without faith but with a modern understanding of the Universe. You are not a Christian because you believe this; you believe it because you are a Christian.
And it shows. Those carols and nativity plays are distinctly unthreatening to the modern secular sensibility. The words of the carols have become meaningless, if charming images. There's a lot about Kings and Kingdoms, reflecting political arrangements that we now consider to outmoded, and indeed illegitimate as anything more than a token figurehead. The Virgin Birth is clear nonsense to the modern mind (What does God's genome look like?).
There is much in the Christian message that resonates as powerfully today as it did 2,000 years ago. But these Christmas trappings get in the way to the non believer. To the believer it might be different - they serve as link to the ancient past. They might help deepen faith, but they will not broaden the appeal. So Christians at Christmas are best off preaching to the already converted, and watching the evolution of the secular festival with amusement.