Pope Gregory I, that is, who in 601 instructed the missionaries to go convert old pagan practices into Christian ones. Evil syncretism? Mixing of paganism with Christianity? The source of elements which must be expurgated from our celebration of holy days? I'm not so sure.
Pagans, after all, were not necessarily anti-Christians. They were pre-Christians. They did not have the full revelation of God in His Word; yet they had the partial revelation of God in nature. They did not yet know of redemption; they did at least know of sin. Just because they did something doesn't mean it was an evil thing to do.
It is God who created times and seasons and days and years. And celebrations, pagan or Christian, are tied to seasons and times. Certain times of year seem more suitable for celebrations than others: the return of life in the spring, the completion of harvest in the fall, the renewed lengthening of the days in the dead of winter. Yes, the pagans may have worshipped their false gods on these days, but the days themselves were important because of the way God made the world.
If one was reaching out to these pagans, what better way to connect with them than to tie into the things they knew, in a manner like that Paul used on Mars Hill: "You celebrate the rebirth of the god in the spring? Well, let me tell you of how the One True God really did come to life in the spring." "You remember the dead in the fall? Well, let me tell you tales of the dead who followed Christ."
Even if a day had been used for out-and-out Satan worship (an unlikely situation, since those who did not know God would also not know about his adversary), what better time to celebrate Christ's defeat of Satan and all his works than on the day formerly devoted to his honor? The devil shouldn't get to claim any days for his own; all days belong to God.
There was, of course, a danger of syncretism. I do hope the monks were careful to draw distinctions; I'm sure some were more so than others and some people listened better than others. However, modern westerners are as unlikely to be lured into ancient religions from practices with a tenuous connection as they are likely to be lured into worship of the Greek gods from reading The Odyssey. The real danger of syncretism today is from pop psychology, feel-good theological fuzziness, and the omnipresent worship of self. That's where we should be concerned. We don't need to fight battles that died out a millenia ago.