Whatever your belief system, y'canna change the laws of physics. This is the day where the sun has reached its southern most point. It's in the sky in the northern hemisphere the shortest number of hours; in the southern hemisphere, the longest number of hours; and in areas south of the antarctic circle, they have been having "midnight sun" for some time now. North of the arctic circle, people have once again had to put their faith in other people telling them about "sun" because they don't see it. The people in the middle get to watch the sun wander from one side to the other, and wonder what a "season" is.
Celebrate! The short days will lengthen, or the long days will shorten. The earth moves, rotates, changes, grows, dies, and grows again. Celebrations include food, good friends, remembrances, and plans for the future. Enjoy!