athiest do believe in something. They believe in what is. They believe in what exists. So atheist believe in the real or reality. That's all.
Most of your beliefs are based on testimony from others. Christians do the same. Who is your mother ? You know who she is based on testimony.
Do you believe that it snows in Siberia?
If so, why? Unless you have been to Siberia, how can you know?
Do you believe in atoms? Why?
But, you know what, as a Christian, I also believe in reality and what is. Jesus lived and died on this earth, and was reprted by hundreds of eyewitnesses to have risen from the dead. You may chose to not believe their accounts, but then you should also not wander too far from your neighborhood, because I shold think you would be afraid of falling off the earth.
I say this not to be insulting or unfriendly, but to point out that your simple encapsulation of atheism is quite problematic.
You should probably think about getting together with other self-professed atheists and working up some doctrine for atheism. Do you see my point?
I think christmas has been co-opted by secularists the same way the christians successfully co-opted the popular pagan celebration long ago. Almost 100% of people celebrate the secular santa version of christmas with lights, presents, decorations, christmas trees, parties, food and candy canes. Christmas has about as much religious significance as halloween. A fun time of year to have some fun, excuse to party, give gifts and spend money. Christmas has evolved and if it ever was a real reglious holiday, it isn't anymore.
I don't know why they don't just declare the federal holiday secular so everyone is satisfied. If various religions want to celebrate their gods on the same day so be it.