Isn't there a religion that celebrate Winter Solstice? The group should be named Freedom from Christianity instead of Freedom from Religion.
Quite a few. The most common in the US is, I suppose, the synthetic religion known as "Wicca" by its illiterate proponents.
"Isn't there a religion that celebrate Winter Solstice? "
Sure. Most of them are pagan religions. However, Christmas also falls pretty close to that astronomical event. In fact, Christmas was set to correspond to the pagan celebrations, in order to help convert the pagans.
These days, in the USA, it's mostly Wiccans who celebrate the solstice. In other countries, other groups do.
The Winter Solstice is also noted and celebrated by many atheists. It's a recognizable event of science, and there's plenty of reason to celebrate the beginning of longer days than nights, especially if you live in northern regions.