Presumably, it was some sort of mid-winter celebration (Yuletide, or something like that?). Anyway, there's nothing unusual about religions adopting previous holidays or holy items and such. Why, the Qaaba (probably misspelt it) was there in Mecca before Mohammed came along.
But of course, while discussion of facts and such is perfectly normal, I think that the discussion of the "true meaning" of something like religion is very dangerous. And, for the same reasons which make it dangerous, it's also utterly pointless - regardless of what religion we adhere to (or not whether we don't adhere to any), each of us has his own notion of the true meaning of such things, and generally feels strongly about it (sometimes, even the atheists).
So, watch your mouth... err, hands.