23.45° Axial Inclination - that's the reason for the season. The earth's axis is tilted 23.45° from the plane on which it orbits. This tilt is for the most part constant as it orbits the sun. That gives us four seasons as each hemisphere alternates between being angled towards or away from the sun.
Jesus is the reason for Christianity, but winter solstice celebrations are ubiquitous across nearly all cultures and religions.
I don't know why I feel the need to point this out, but I suspect it has something to do with the apparent increasingly outspoken Christian voice here in the Southeastern United States. "Remember the reason for the season!" they keep telling me. So, okay. I did.