Winter season celebrations have been around seemingly for as long as our species has been around. In some sense Christians are late-comers to the party. It really doesn't make sense to try to hoard the festivities especially in light of the fact that the ideals Christians frequently claim include compassion and empathy. Why get hung up on labels? Merry Christmas, Happy Holidays, Seasons Greetings,... Whatever you want to say, or however you want to celebrate shouldn't be an issue. I happen to be a "Winter baby." I was born on the Winter Solstice and brought home on Christmas day. Atheist or not, this is still my holiday(s) as much as it can belong to anyone or any group. I'm willing to share the joy.
A few related snippets from around the web:
‘Taking Christ Out of Christmas’ Is Nothing New, Historian Says
Away with the manger, in with the solstice!