To begin with, a large number of pre-Christian holy days and festivals were held around the 25th of December, usually some days earlier, on the 21st or 22nd, when it's the Winter Solstice, the longest night of the year. A non-exhaustive list includes the Saturnalia (Roman - when slaves were served by their masters for a day), Yuletide (Druidic - when the old Sun died in order for the cycle to begin anew), Mithraic Celebration of Sol Invictus (actually on the 25th, a bull was sacrificed in order for the young Sun god to be born), Shabe-Yalda (Iran - when people gather at home around a korsee (a low square table) all night, to tell stories and read poetry. They eat watermelons, pomegranates and a special dried fruit/nut mix. Their are tons of festivals to be found around the world from Brazil, to Native American Tribes, to India and so on and so forth (you can read more here).
The oldest surviving evidence of Christmas Day as a proper holiday (and celebration of the Birth of Christ), dates back to 336 A.D., in Rome, mentioned in 354 A.D.'s Philocalian Calendar (see here, page 20). Of course, it has been argued and debated ad nauseeum, that the historical figure of Jesus Christ was not in fact born in December, but somewhere closer to April and of course, he was not born in the year "0" but following that numbering, probably around 3-4 A.D. That much is probably true, especially the falsity of the 25th of December, given that, originally, this holiday was created as a "festivity merger" and later on, during the Church's "hostile takeover" of the older religions, as a counterbalance to one of the most important pagan holy days.
Given what I write, one might think that I am making a case against Christianity. Not at all. In fact, I DO believe in the one God and call myself a Christian. That said, it doesn't mean that I agree with the trappings of organized religion such as it is today or such as it was in Byzantium, after Emperor Constantine forced the Synod to create "one proper faith", to unite his people. Faith and religion are two very different and nigh on unrelated things. Hence, I dub myself an Unorthodox Christian (for, surely, most priests would have excommunicated me). Don't even get me started on respecting other religions and believing that, in fact, Science and Faith don't have much to argue about, at their core, because some would be inclined to exorcise me or treat me to a round of psychotherapy sessions.
Besides, I digress: our subject is Christmas and all the other related holidays. As you have seen, there are quite a few queries and more than a few ambiguities on the whole subject matter. However, in almost all the aforementioned celebrations, there are three common axes: hope, kindness towards and fraternity between people, as well as between people and our small blue world, be they Zoroastrian, Wiccan, Druid, or indeed Christian. These things are the only ones to keep at heart, for everything else - and I DO mean EVERYTHING - in the end is ashes and dust and our differences, quarrels, territories and wars, merely smoke and mirrors.
Merry Christmas and a Happy Holiday to you all,