Medieval man fused existing elements of pagan midwinter rites with the developing theology of Christmas in an appeal to the senses of both sacred and lay.
As the European winter deepens the course of the sun gets nearer the southern horizon until it seems to stand still for a few days, before slowly rising again to usher in the spring. Ancient peoples were of course aware of this phenomenon and the Romans, from their words for 'sun' and 'stand still', called it solstitium, source of our word 'solstice'. By the Julian calendar, the system of months and days we still use (first devised in 46 BC on orders from Julius Caesar), the winter solstice was originally dated December 25th.
All religions and philosophies draw on analogy; so much so that some medieval philosophers, inspired by Plato, saw the whole universe as a vast web of analogies, of hidden likenesses between one thing and another, likenesses capable (they said) of leading the gross human mind from the visible to the invisible, preparing it for the deepest truths of religion. The number of such analogies must be infinite. But the most obvious of all remains that between the sun and God; or rather, one sort of God: one who is, like the sun, a universal source and sustainer of life, and without whom all is blackness and cold. The analogy is so plain that, like the solstice, it was generally noticed by ancient peoples, and the worship of the sun as God is consequently a commonplace of ancient religion. In the Roman world the main form of it was the cult of the 'Unconquered Sun', a keystone of Mithraism, in its turn a leading contender for the devotion of Roman subjects in the late third century AD. In 274 – and when else than on December 25th? – an Emperor declared Sol invictus principal patron of the Empire.