AH-HAH!!!
I see the problem!!!
Clash has confused ATHEIST with SATANIST!
Yes, THERE is the issue.....
A SATANIST believes there IS god, and every action is done to oppose him.
An ATHEIST does not believe in god, and thus, does not measure his actions as to whether or not they have anything to do with your invisible friend.
A SATANIST should not celebrate christmas, since he DOES believe god/christ exists, and opposes them, celebrating the birth of christ would be hypocritical.
An ATHEIST can celebrate christmas or not, it's up to him. Nothing of importance happened on that day in history to the atheist, nor is associated with that day, so whether he celebrates or not is strictly based on whether he wants to participate in the secular holiday or not.
See, the difference is, a SATANIST cares and opposes what a christian thinks, and ATHEIST does not. That Easter is a celebrated as a religious holiday is significant to you and the SATANIST, but matters not at all to the ATHEIST.
AND, HERE IS THE KEY:
If an ATHEIST is deciding whether or not to celebrate a holiday, ALL HE IS CONSIDERING is whether or not he wants to enjoy the festivities and social gatherings or events associated with it or not. NOTHING MORE OR LESS. That the holiday may (or may not) have any religious significance to christians, hindus, buddhists, pagans, jews, or muslims doesn't even factor into his decision in ANY WAY.
IOW, an ATHEIST 'lives his belief' (using your term) merely by NOT INCLUDING 'GOD' IN HIS DECISION AT ALL.
A SATANIST, on the other hand, in deciding whether or not to celebrate a holiday, HAS TO CONSIDER whether or not god is involved, and how. Then, he decides whether he should celebrate the holiday as it is, or have a special version of the holiday to oppose god.
IOW, A SATANIST 'lives his belief' by ACCEPTING GOD AS REAL AND FACTORING THAT IN HIS DECISION.