Under the Julian calendar, the winter solstice was fixed on December 25, and this date was also the day of the popular Roman holiday of Saturnalia, in honour of Saturn, the god of agriculture; which was later superseded by Sol Invictus, a day that bundled up the celebration of several sun based gods into one easy to manage festival.
As Christianity began to take hold across the Roman empire and beyond, the date of when to celebrate the birth of Christ became a bit of an issue, with several different dates proposed.
It wasn’t until 350 AD, when the then Bishop of Rome, Pope Julius I, fixed the official Christmas day on December 25. Unfortunately, Julius, I didn’t show his working out on how he reached this date; some scholars later suggested that it was calculated as nine months after the Annunciation (March 25), when the angel Gabriel is said to have appeared to Mary and told her she would bear the son of God. Whatever the reasoning, it is clear that, just as key pagan sites were being chosen for new churches, so too the date was chosen with the intention to catapult Christmas into becoming a major festival by placing it over the pre-existing pagan festivals.
A little-known fact about Christmas Day is that it was once banned in England during the 17th century. From 1647 to 1660, under Puritan rule led by Oliver Cromwell, Christmas celebrations were outlawed because they were seen as pagan and frivolous. People were expected to treat December 25 as a regular working day, and festivities like feasting or decorating were suppressed, though some continued in secret.