I reckon that people found the 'fixed' points, the nadir and the zenith, the solstices and the equinoxes, and tied their year, their celebrations and their lives around those.
Seasons are too mutable, this December warmth is a case in point. But the day length, the time of light and dark, those are not.
So, the Solstice marks the shortest day and then the light returns, and folks have a fire and a party to encourage it along
In to that they tie their hopes for a good year to come as well.
We do know that in the past (I believe that the Jewish folks still do) people thought of a day as from dusk to dusk…..like us starting the clock again at midnight I suppose. One in the 'morning' is generally still dark o'clock
So, is it really this longest dark night that we should light up ?
M