You misunderstood me. I said that if it was to be universal, then there was things to take into account, because the world is what it is. I do not think that it must be universal, even if I like the idea of gathering around the solstices. Christian people can still celebrate Jesus at the same date. It is not contradictory.
By the way, what you criticize here is your own approach : "The modern celebration is the birthday of Jesus", which is not related to reality. Many people in historically christian countries celebrate around an evergreen coniferous tree, but do not celebrate Jesus. And there are many people, which are not christian at all, in what we call "christian countries" and in other countries, which are muslim or jew for example, that do not celebrate Christmas at all, neither do the celebrate the Sol Invictu.
The principle of universality is put forward by catholicism. The word "catholic" means "universal" in greek. So if you are against the principle of universality, I deduce that you are against the catholic church. I won't blame you for this.
As an atheist, I believe that if there is something to be called "universal", then it can be found be science, science of the matter and science of the mind. It is possible that we do not find anything universal except that we are all human beings, and that culture is only a matter of choice.