"You're assuming that neolithic humans practiced religion and the solstice has always been observed for religious reasons. Neither of those is proven. For as long as humans have marked the seasons there have been celebrations of their passing, and they were not all religious in nature."
So what is your theory? Neolithic humans woke up one day and said "Hey, let's celebrate a random point in time every year. But let's do it 'just because,' without assigning an spiritual or cosmological import to said random point in time." Absurd.
"That doesn't mean it was the only way humans could have developed."
Maybe, maybe not. The point is that it is the way that humans did develop and you cannot escape from that.
"We're shaped in many ways by Shakespeare and the Enlightenment too, but that doesn't make either of those things religion."
I never claimed otherwise. But if someone were to going around trying to promote a movement claiming to utterly erase the influence of the Enlightenment, surely you would find the claim suspect. How much more suspect is the idea that we can simply move beyond something much more pervasive like religion?
"An atheist saying "bless you" is completely devoid of religion. To that person it is simply a social custom. You read far too much into it."
You don't read enough into it. It doesn't matter what the expression means to the person using it. What matters is that religion is so written into the language and customs and institutions of the world's cultures that even militantly antireligious individuals invoke religion without even noticing that they do so.