I personally don't have anything against having days of thanksgiving. There are two example in the Bible of two days of thanksgiving in addition to the Holidays of the Torah.
One is Purim (
Est 9:26-32) and the other one is the Feast of Dedication (Hanukkah) (
Jhn 10:22).
The Bible doesn't condemn them.
I think, if you wish to celebrate it, and you do it keeping the significance in mind and behave properly in godliness you are fine.
Hanukkah, fortunately, has kept its seriousness and sobriety in general.
Purim has become a very wordily event in Israel, however, the ultra-orthodox Jewish still celebrate it and try to do it the right way, with joy and worship to God.
So I think, if you are going to celebrate Christmas as a thanksgiving day with joy, at least do it in an acceptable manner and stay away from "symbolic" ornaments, Christmas movies, and any pagan ways (trees, elf, etc...) but instead in simplicity and in Spirit and Truth. Instead decorate yourself with good works that day and your house with verses of the Bible for your own benefit, and sing hymns purely exalting God as you do in your devotionals, and bring the story to remembrance. If you can't stay that way, better drop it all together and turn it into a Family & Friend day and enjoy the time off.