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1.
An annual holiday around the winter solstice that is celebrated by Atheists. For this holiday, gifts are exchanged out of goodness and out of love, and families come together. Similar to the Christian holiday, Christmas, minus the religious aspect.

Also celebrated by Agnostics.
The entire Davis family came together this year to celebrate Atheistmas. Everyone gave each other gifts and were happy to see each other.
by Jackson720 December 24, 2009
 
2.
A holiday created for Atheists by author Kevin Moyers in his book "Brain Farts", which was published on May 15, 2009. Atheistmas is to be celebrated annually on August 11.
I can't wait to see what gifts I get for Atheistmas!
by unleashed52 April 01, 2011