A few years ago I stopped saying “Happy Holidays” as my default and switched to “Merry Christmas.” The main issue for me is that I didn’t want to get hung up on a name. As someone who doesn’t accept that Jesus Christ was the Son of God I don’t celebrate the season for that particular reason. But winter festivals have been common across the northern hemisphere long before Christmas, and if you dig through the literature from many fundamentalist Protestants you’ll find they’ve done a good job cataloging the exact pagan antecedents of many Christmas traditions. If I lived in a nation which was mostly non-Christian I would celebrate the same communal holidays which symbolize the importance of such festivals in a properly functioning society, the name would simply be different.
As a person without supernatural beliefs the holidays have no nominal connection to what is “up there” for me. Rather, they are about what is “down here.” We don’t live in utopia, but for this small period of the year we put aside our concerns, worries and grievances, and celebrate all that there is to celebrate.