Astronomically, it's the winter solstice, where the north pole is furthest from the sun and gives us the shortest day of the year in the northern hemisphere. There was a good post last weekend about the timing and the dates:

https://greatawakening.win/p/11R4XB98Dd/big-signs-in-the-sky-this-didnt-/

Not saying I believe in astrology, but biblically celestial movements have been used to mark times and seasons.