To which, my response is a resounding 'So what?' If we didn't celebrate the birth of Jesus in December, we'd just do it in June or some other date that someone decided was as good as any. And so what if we have kept on a few old traditions. If you want to celebrate Yuletide or the Winter Solstice or Hogmanay or me and animal's anniversary, knock yourself out. I choose to celebrate the birth of my saviour at this time of year.
So, to answer the question: I've celebrated Christmas all my life; it's just what we do. In this country we need a bright spot in the middle of the depressing wet winters we usually get, and Christmas is it. It's an excuse to get together with loved ones, and at least be civil to those we don't love quite as much. And that's good; peace and good will to all man is a good thing, whatever you happen to believe in, God-wise. Family get-togethers are... well ok, they can be a bit fraught, but it's once a year. If I only saw my sister once a year we get on far better I expect...
So Christmas is a good thing, and I don't have any issue with non-Christians celebrating it. After all, I celebrated 24 myself as a non-Christian...
But, dates aside, as a believer it obviously has a deeper meaning now. And it doesn't matter that Jesus never said 'celebrate my birthday'. He didn't say don't celebrate it. Why would the event be described in such detail in the Bible if it wasn't important?
I've posted this here before, but this song pretty much sums up the religious reasons why I celebrate Christmas:
Merry Christmas, and I'll be back next year.