A few years ago there was a lot of discussion that terms such as “Merry Christmas” should be banned from public use. The City of Toronto forbade it. It appears that the leading atheist advocate Richard Dawkins is more than happy to wish everyone a Merry Christmas.
So divorced has Christmas become from religion that I find no necessity to bother with euphemisms such as happy holiday season. In the same way as many of my friends call themselves Jewish atheists, I acknowledge that I come from Christian cultural roots. I am a post-Christian atheist. So, understanding full well that the phrase retains zero religious significance, I unhesitatingly wish everyone a Merry Christmas.