Its opposite also comes up: If atheists don’t believe in Christ, then why do they celebrate Christmas? Christmas is a Christian holiday, after all.
Except of course, Christmas is not a Christian holiday. Well, the name is. A holiday surrounding the Winter Solstice exists in many cultures past and present, most of which aren’t Christian.
Christmas is a Federal holiday in the USA (I Amendment clause against establishment of religion can go hang itself). However, that is actually only a recent development (1885, 109 years after the American Revolution).
If you look up the history of Christmas in the United States on the official organ of the government, Voice of America, you’ll find the traditional whitewashed history of the holiday. The History of Christmas in America - VOA - learningenglish.voanews.com
In the first half of the 19th century, Christmas was a very different kind of holiday than it is today. People did not have a set way of celebrating. Christmas was ...
However, like Thanksgiving, the national legends surrounding Christmas are not the same as the actual history of the holiday.
In the early 1600’s under Congregationalist (Puritan) theocratic rule in New England, Christmas was outlawed. Any outward celebration of the holiday (considered pagan in nature) resulted in a fine of fifty shillings (a hefty sum then). Because New England in the early colonial period had such an effect on the rest of the colonies and later the nation, the holiday was not celebrated prior to the Nineteenth Century in the USA. In point of fact, the first Congress under the Constitution met for its first session on Christmas Day.
Some colonials in other areas such as New York or Pennsylvania had adult-only celebrations, but by the time of the American Revolution, they were entirely dropped, seen as unpatriotic. (Christmas celebrations were considered then an English tradition.) One point which shows that is George Washington’s famous crossing of the Delaware River, which took place on Christmas Day to engage the British in the Battle of Trenton the following day.
It was only in the 1820s that the ultra-religious started to lose their grip on government (thanks to Separation of Church and State outlined in the Constitution) and people started shopping for gifts. Around that time, churches started decrying commercializing what they viewed as a religious high day. The phrase “Merry Christmas” (the endless drumbeat of conservatives today claiming evil secular humanists prohibit them from saying – not – was only popularized only after Charles Dickens published “A Christmas Carol” in 1843.
Christmas trees only appeared as a regular feature in homes in the 1870’s.
The War on Christmas had already been waged since colonial days by Christians, with Christians winning a battle in that with the declaration of Christmas as a Federal holiday in 1885. (This for over the first one hundred years of the nation’s history, the nation did just fine without Christmas.)
Commercialization of the holiday with gift-giving and such only really took hold in the Twentieth Century, with advances in marketing and mass-media.
All these things (Santa Claus, Christmas trees, gift giving, &c) have no basis whatsoever in the Bible or Christianity. (Neither do Starbucks holiday cups.)
So why do atheists celebrate Christmas?
Well, we like the idea of gathering with family or friends, gift-giving, hearty meals, exchanging cards or sayings of good will, just like Christians do. Christianity does not have a patent on these things, and in the history of our nation, Christians actively opposed them (or made them illegal).
Like many other things, Christians were dragged kicking into more modern views by secular thought, not their own thoughts.