Why shouldn't I?
The real question is why do Christians insist on celebrating a Pagan Holiday, when there actually never really was a Christ in Christmas at all.
When you set up your "Christmas Tree" do you think of all the Pagan people who celebrated the Solstice in just the same way? And gave each other gifts?
The Christian Christmas is a fabrication based on a falsehood, and I have no idea why Christians celebrate a pagan holiday when they know, or should know, that it's not the birth of Jesus at all and actually based on pagan beliefs.
It's like basing your entire belief system on truth and inerrancy, and then going ahead and celebrating the 4th of July in August
The holiday celebrated on December 25 was being celebrated long before the Roman Catholic Church decided to make December 25 the day to celebrate the birth of their half god-half human deity. It was the Roman Saturnalia. It was the Roman Festival of Sol Invictus (the Undefeated Sun God). It was the Pagan festival called Yule ( as in Yuletide and bringing evergreen trees into the house to decorate).
The earliest Christians believed that Jesus was born on May 20, or possibly March 25. It wasn't until about 378 CE that Catholics decided Dec. 25 would be the "birthday" of Jesus.
So much for that. Now, as far as I am concerned, if you Christians want to celebrate the pagan feasts of the solstice, the Festival of Sol Invictus, the Yule or the Saturnalia and PRETEND it is the day that your half god-half man was born, it's okay with me.
But I really think you should at least be aware of THE TRUTH behind your "birthday celebration".