LOL nothing like a loaded question! If I had a few months to go on and on... I might be able to answer it half decently.
There's a lot of debate about how the various Christian celebrations came into being. It seems to be fairly accurate to say that a lot of Christian celebrations were celebrated on the same day as pagan ones -- or as Jewish ones (think of Easter and Passover). But why is this? That's where it becomes difficult to say. Some Jesus-followers probably did just want to take the festival and "slap a Jesus label on it". Others likely wanted to change something "evil" into something "good". Others just wanted an excuse to party. Some would celebrate (for example) Dionyses and Jesus in the same celebration, and saw no problem with doing so. Put it this way... why do people -- even non-Christians -- celebrate Christmas now? There's a million reasons, and if you put them all together, you end up with Christmas as it is these days, with Santa and Baby Jesus sharing a spot in the mall (and occasionally a menorah for Hannukah around, too!) Something similar probably went on in the early Christian communities. Then again, the Roman Empire was so culturally diverse, it's really hard to say what came out of where. And when it comes to history, the answer is never simple anyways. That's the best answer I can give you in such a short time... basically... who knows?
But yes, this was a case of pop religion influencing what would later become official doctrine.
With regard to pop religion vs. official religion in the modern day and age... is there still a difference? Sure. Is one "right" and one "wrong"? Nope. Just two sides of the same coin. Sometimes official religion adapts the un-official views, and sometimes it's the other way around. Religions are all always constantly evolving, because ultimately, it's the people who are at their core that are it's life and breath. Look at the religious phenomenon of the "Left Behind" series of books. When they came out, they were absolutely destroyed by theologians who said they were "terrible theology." Now, a lot of theologians -- not necessarily the same ones, but a lot of them -- are teaching the end-time vision portrayed in those books. In some circles, this has become the "official interpretation." In others, it remains a pop phenomenon.