The origins of the modern festival of Christmas
Midwinter is not a pleasant time. It is cold and dark, especially as you go further north, so people needed to boost their spirits as the shortest day approached. Across Europe a great midwinter Pagan festival developed to cheer them up. The people of the North called it Yule. The Romans called it Saturnalia.
At Yule the Saxons and Norsemen countered the cold and dark of the northern winter with a huge feast where they ate roast pig cooked over the Yule log, which was dragged in from the woods to burn on the hearth. They drank mead and ale and kissed under the mistletoe [or more] All this was to emotionally strengthen them for the last half of the grim winter period until the first signs of spring. In the meantime the Romans had their own celebration, Saturnalia, and being Roman, they celebrated with rowdy, randy revels. This is where the Church came in. The priests were unhappy at the behaviour during the feast, so decided to make it Christian it by setting Jesus' birth on Saturnalia day, the 25th December, to encourage people to celebrate more religiously. Linked to this was the belief that a Pagan deity, Mithras, a rival to the Christian God, was supposed to have been born on 25th December, so the Church could muscle in on Mithras' celebrations as well.
The strategy worked to some extent, and the name Christmas [Christ's mass] developed, but the Pagan aspect never went away. The Yule log is remembered, even though it is now chocolate. Kissing under the mistletoe, a memory of Pagan fertility rites, still happens. The Christmas tree was an old German Pagan symbol that was brought to Britain by Prince Albert. Its evergreen status was a sign of everlasting life in the cycle of rebirth. Massive feasting is still the norm.
In fact, nowadays we have the remnants of the Pagan festivals, with a superficial [for some people] Christian religious belief/mythos covering it. Old festivals never die, they merely evolve with the times