Some of my atheist/agnostic friends do not celebrate Christmas because they think it is a made up pagan holiday. Some of my fundamentalist Christian friends do not celebrate Christmas because they think ii is a made up holiday appropriated from pagans. Go figure!
If you have already made up your mind about "the reason for the season" then you will already disagree with what I am about to write. The reason for the season is celestial mechanics. People have likely been celebrating around the winter solstice long before recorded history. Most researchers now believe that Stonehenge is a giant celestial calendar which produces a "ho hum" from most of us who are used to calendars and clocks. But, imagine this, there are no clocks or calendars. Every year you notice that at certain times of the year the days start getting shorter. At some point you notice the days start to get longer. Now imagine the power you would seem to have if you could *predict" when that change would happen. Enter Stonehenge (and I believe a number of other prehistoric sites that now seem to have been aimed at predicting the start of seasons). You can now predict when the days will get longer and celebrate the end of the darkest nights.
So...I think anyone who wants to should celebrate "Christmas" in any fashion that they believe is culturally appropriate (no human sacrifices, please ;-)) like hanging out lights, or putting up trees and decorating them, or singing Christmas carols (hmmm...even some carols, like "Deck the Halls" seem to have been appropriated from other reasons to celebrate), or sitting on Santa's lap, or watching "Rudolph the Red Nose Reindeer", or buying gifts for their family and friends...or...