The ancient Romans, like virtually all peoples of the time and since, celebrated the winter solstice. Sol Invictus, or Unconquerable Sun, was on December 25 by the Julian calendar.
Christianity, for the first millennium or so of its existence, was tasked with a sales campaign to convert the pagans. A brilliant idea was to NOT cancel out the pagan feast and holiday celebrations--that had been tried with the Celtic and Germanic tribes and had failed miserably --but rather, to re-label them. Thus, the Jesus/Mary/Joseph myth was plastered directly onto the day of Sol Invictus.
This had great utility. Everybody expected a winter solstice party, and few really cared what idols or statues or symbols were used. It was a time to light the bonfire and get roaring drunk. Sure, a priest would stand up and pontificate, but like the tribe had done with their shaman before the priest arrived, they just laughed and handed him a beer.
In lands/communities where the authorities cared which religion the natives subscribed to, the people could just say that they drank eight tankards of ale to, um, celebrate the birth of Jesus. If an older guy interrupted and said, "I thought we were paying tribute to Thog the Glorious," he would be shushed and given a piece of pumpkin pie.
We all feel this relief, whether consciously or subliminally, that the days will now grow longer, not shorter, and spring is on the (sometimes distant) horizon. The weather in most of the world is actually crappier post- than pre-solstice, but we can at least see the light at the end of the tunnel.
And three cheers for Thog the Glorious!