American Thanksgiving (celebrated this coming Thursday, Nov 26) is a civil holiday with vaguely religious roots. Over the decades churches have co-opted it and turned it into a major holy day (for Evangelicals, at least). Now, when Americans treat it like the civil holiday that it actually is, some Christians sputter and get offended as if they actually own the day because they co-opted it.
A similar thing happened to the ancient pagan solstice festival. Christians co-opted it and associated it with the birth of Christ (which was likely in August and almost certainly not in December). Recently Wicca and other earth religions have been trying to take back their own festival, but with very little success.
It’s what Christians do. They take the stuff of nature and culture around them and see Christ in it. (“By him all things consist” Col 1:17). Sometimes those events turn into feasts and fasts. It’s a remarkable process of seeing the “natural” world through the eyes of faith.