Earth's 23.4° Axial Tilt is the Reason for the Season

Both literally and metaphorically.

Earth's tilt physically causes the season, including of course, winter. When the northern (or southern) hemisphere is pointing at the sun, the longer days and more direct sunlight make it warmer. When it points away from the sun, the shorter days and less direct sunlight make it colder. Hence the seasons. The solstices are when the Earth is pointing most directly towards or away from the sun and they mark the transition between days getting shorter and days getting longer.

Earth's tilt is also the cause for the holiday season. Wikipedia lists no fewer than 36 celebrations related to the winter solstice, from cultures all around the world. And it's hardly surprising when you think about it. Even before the discovery/invention of the calendar, I'm sure it was clear to people that longer days were warmer days. Also, when the sun was up determined when you could go hunting and get things done. So the reversal of the shortening day must have been extremely important.

Among that list from Wikipedia are some holidays you might recognize. Saturnalia, Yule, Hanukkah, Christmas. Christmas isn't really any different from the others, and most of our celebrations of it don't have much of anything to do with Christianity. The Christmas tree for example, has roots in a pagan celebration.

Also, the date of Christmas has nothing to do with the story of Jesus's birth in the bible. The bible never gives a date, and it says that the shepherds are in the fields with their flocks, which they would only be during a warmer time of year. Most likely, the early church selected the date of Christmas to match a Roman holiday.

Jesus isn't the reason for the season, Earth's 23.4° axial tilt is.