If we are discussing purported "fact", I would say, that, what's been peddled through the nativity story as part of this (traditionally, although not in my part of the world) midwinter festival/celebration, is the lie that the alleged historical figure "Jesus" was born at this time of year. This has no evidence to support it.
This is my issue, that what it actually is, is a pagan celebration "Yule" that was, in itself, a very controversial and at times contested celebration, throughout our not-very-distant past.
That this "Yule" was highjacked by Christianity, has far more to do with politics, than it does with an historical event of a rebellious spiritual leader who stood up to the Roman rule and the profiteering Jewish hierarchy of his time.
I like to assign my own "fact-based" meaning to the event. To me, the nativity story is simply a symbolic one; a celebration of the basic building block of society and civilization itself, the family unit - man, woman and the birth of a family through procreation, the birth of a child. It's a beautiful and wonderful symbolic story of adversity, astrology, biology, healthy rebellion and multidimensional awareness, paranormal activity and joy.
I don't believe the historical figure we know of was born at that time, but I also don't believe that those stories came out of nowhere. I don't believe that early Christians were getting themselves publicly murdered for no good reason, or that their faith was baseless.
I'd like to learn more about the pre-Christian yule festival and get back to a more fact based informed family get together.
Currently, my family is in total flux about Christmas, but I always use it as an excuse to lavish my children with gifts and luxurious food. I never taught them any lies about it though, as, for me, finding out I'd been lied to about it, as a child, was a time of a deep sense of betrayal.
The children born to me are now part of a "broken.family". Their father and I separated 8 years ago and he maintained primary care of our then teens and young adults and he never bothered to celebrate the event, at all (he, I believe, is also on the spectrum).
Many of our children have a lot of sadness and disappointment about the event so I'm now trying to remedy that, and instead of "Christmas" those members and I will get together for "Solstice".
I believe in trying my best to remedy and repair a much maligned and oftentime damaged moderntime unit of society, that of the family, and that is what is meaningful to me about the nativity connection and the day of 25 of December.
What this proposed inverted cross gesture signifies? Simply some teenage type rebellion against the "Mummy and Daddy" State/Religious political Romanization of what was once a time of cheer and much meat eating in the middle of the coldest time of year.
Not without validity or merit, necessarily, just a symbolic gesture of a dissident. I quite admire dissidents, often, but "Satanism" simply does not impress me at all.