Merry x-mas.

From a militant atheist.

All your stupid christmas, chanuka holidays are derivatives of the fundamental
state: the shortest day of the year, thanks to the earth's tilt with respect to
the solar orbit. Likely due to the Moon Event.

Anyway it makes for wider habitable range, albeit with seasons. More extreme
the higher latitude you go.

As hunter gatherers or (as much) farmers you care about the photoperiod.
Modern folks just synch to Hallmark.

Modern man is a farm animal. First farmers found productivity, then looters found
farmers, then armies, then politicians / wordsmiths / forebrains
farmed the armies, and here we are.

But back to the Cycles: we are not immune. I've avoided SAD via not wearing sunglasses
(plus a few psychopharms) this season. But I await the elongation of the day
that starts on the shortest day. Derivatives of cosines are sines.

I even acknowledge ---celebrate is too strong--- the mid-season days when the derivatives
at at their highest. Those are also the equinoxes, because of cosine-sine properties.

The sine wave (aka cosine, phase shifted 90 degrees) is what you get if you cast
a shadow down on a rotating arm. Its what you get when a sharper wave disperses.
Its what you get from simple harmonic oscillators. Guitar strings. Organ pipes.
It is Optimal in ways, eg smooth. And its derivative is itself, phase shifted.