We've all heard a lot about the "pagan" origins of Christmas. This is of course not as much scandalous as it is true. But I think it fair to say that Christmas was the result of a strategic appropriation by Christians (for which Bede gives us a clear record) rather than hostile infiltration by pagans. Pre-Christian cults such as that of Isis and Bacchus didn't just roll over and play dead. They had to be intelligently subverted, in the same way that other forces try to intelligently subvert what remains of Christian culture today (see below).
But when it comes to really understanding current Christmas traditions, the key may not be as much the ancient pagan past than the recent Victorian one. It seems the War on Christmas didn't start with the ACLU but with Reformed Protestants, who nixed it because it smacked not of paganism, but popery. In England Puritans outlawed it under Cromwell, a prohibition which continued in Boston, where from 1659-81 one could be fined for its celebration. Thanks to England's Oxford Movement however there was the nineteenth century Victorian reappropriation, which these United States (now sufficiently individuated from the motherland) were prepared to pick up on.
Perhaps America's cozying up to Christmas may also have had something to do with the literal war on Christmas of 1776. German Lutherans were of course much less priggish than the Reformed about things smacking of popery, and more than one historian has surmised that the Teutonic tradition of robust Christmas celebration may have led to Hessian mercenaries being easily overcome after Washington's crossing. Should this be true, then our country is the happy result of Christmas hangovers.