If you don't believe in God, how do you celebrate Christmas?

I used to celebrate the Winter Solstice instead.

What we now refer to as Christmas has a long heritage drawn from pagan religions, e.g., the Roman Saturnalia, the Yule celebrations of pagan northern Europe. (One of these customs is the Christmas tree). The Persian god Mithras was born on Dec. 25. It is instructive to recall that there are no biblical commandments to commemorate Jesus's birth. The celebrations are cultural, not religious, and certainly not specifically Christian. A Christian can MAKE them religious, of course. That is the person's choice, but it is not the history of the holiday. But as to your question, I celebrate by getting together with family and friends. Those who live too far away, I send them cards or call them. Another year goes by and I want to catch up with everyone.

If you do believe in God, why do you celebrate Christmas? He wasn't born on Dec. 25th. But it is the birthday of many Pagan gods. The Bible doesn't say to celebrate His birth. The Bible does say that we are to "divorce ourselves" from pagan traditions and it says not to bring a tree in the house and decorate it like the pagans do. You have to figure if pagans and non-believers celebrate it, it's not for us Christians, we're supposed to be different, set apart from the world. I don't do Christmas anymore.

I disagree that Christmas is a season "for Christianity." Christmas is a federal holiday, and there is certainly enough of the season that is completely secular in nature to appeal to the non-Christian. I don't celebrate Jesus' birthday on December 25 because He wasn't born then, but that doesn't mean that I don't love the lights and the decorations.

I don't think there is any hard fact established that Jesus was born on 25th December. I am not a Christian and even I celebrate it, because it is a cultural tradition coming down through the centuries. 25th Dec was chosen for celebrating the advent of Christ only because there was already an established holiday on that day, otherwise they wouldn't have been able to make people follow this new religion. Christmas tree is also part of this cultural celebration, as stated correctly by @msiduri

Before Christianity there was another faith and way of being. I celebrate that partially and cause my Mom is with us now, more sentimental Christmas stuff.

Not everyone who believes in God is a Christian and many don't celebrate Christmas at all. The meaning has got lost to many anyway because Christmas has become so commercialised.

we all have our way in celebrating Christmas.

Those who do not believe in God or Christ are either atheists or members of other religions. They don't celebrate Christmas . . If you invite them, they come to celebrate what you got on the table.