Yeah, in that case, I would capitalize “Winter Solstice” if referring to the quasi-religious holiday, not just the astronomical occasion. I see no reason why the “w” in “Winter holiday season” should be capitalized, though. And the edit with “a very Zombie Christmas special” has bizarre capitalization, too. I would probably opt for (if I were to construct such a sentence): “It could even be ‘A Very Zombie Christmas’ special, if that’s what you’re into!” There are other options I’d be fine with, but the way it’s capitalized in the OP doesn’t make sense to me. However, I would not make a stupid zombie reference in my copy.