In short, Laplace explained how Earth, Gaia, Tellus or whatever name you give it, came to have its currrent seasons, including the shortest daylight of the year in the Northern Hemisphere. He explains the reason behind all of these religions having major celebrations around Dec. 25.
That said, that was a bold enough statement. Post Reign of Terror, the equilibrium in France, and in general, was swinging back away from freethought in general and definitely from outright atheism.
And, starting with this point, from denizens of the northern part of the Northern Hemisphere possibly having SAD 2-3,000 years ago, just like today, there's good secular reasons to celebrate the solstice, and for taking a holiday for it, even if it's piggybacked as part of Christmas Day.
Besides Laplace, we can celebrate the technology that created light boxes to battle solstice-centered Seasonal Affective Disorder. Or the antifreeze the helps let cars run in winter. Or the human storytelling ability that developed narratives about gods, narratives centered on the solstice. Or, the historical-critical and literary tools that show today how those myths developed.
And, now, to tie celebrating Laplace into a bit of recent Gnu Atheist drivel ...
Or, you can be a Gnu Atheist Grinch about the idea of being even remotely associated with Christmas, as is Tom Flynn of the Center for Inquiry. But, to focus on Laplace here, I've got my thoughts about Flynn here.
And, per the commenter, the primary reason I edited the original end of this post was NOT because of "sniping." In fact, Flynn's blog post was in part what shaped some of the details of this one. And, if he doesn't even like our calendar because of how weeks and months are named, I doubt he will see any "light" in the future.