I don't, actually. At least not in any meaningful sense.
I don't think that Jesus was anybody special. In fact I'm not even completely convinced he ever existed at all. I see no reason to celebrate the non-anniversary of his birth.
But I do have a Christmas Tree up in my living room, and I'll be having a turkey-dinner with family on the 25th... probably saying "Merry Christmas" a lot and exchanging gifts.
To the casual observer, that may seem something of a contradiction. And I can certainly understand why that might be the case.
Thing is, I was raised a Christian. Growing up, Christmas was always an important family event... everyone would come for a big family lunch with all the presents and crackers and fruit pudding and all of that. Other than the obligatory church service we would attend in the morning, there was next to nothing about the Christmas festivities that had anything to do with Jesus. He simply didn't feature.
Most of my family have drifted away from Christianity in various directions since that time. But we still get together every year for the sake of getting together. Christmas, like Easter and family birthdays, is an excuse for us to set aside our usual daily routines and enjoy a few hours together as a family.
Not that we really need an excuse, but it's convenient to have one.
Christmas was never really about Jesus, at least not for my family. And now that we don't bother with the church service bit, he's been removed from the whole thing as an unnecessary accessory. The only real contradiction in celebrating Christmas as a non-Christian is in using the name "Christmas".
But since that name was only added onto a pre-existing solstice festival by Christians many years after the alleged birth of Jesus, I don't feel too bad about it. I have no guilt about co-opting a holiday that had already been co-opted from someone else.
Probably the best articulated description of how I feel about Christmas is the song "White wine in the sun" by Tim Minchin. In the festive spirit, I present it here for your enjoyment: