Today, December 21, at 6:12 am EST, was the winter solstice. The time in the Earth's orbit where the Northern Hemisphere has its maximum tilt away from the Sun and the Sun is at its lowest in the sky. It's also the first official day of winter.
Why is it not the coldest day of the year? It is, after all, the day with the least amount of insolation (incoming solar radiation). Here in the Hudson Valley, it's currently raining and almost 50° F outside. It's because temperatue lags behind insolation. The winter solstice is not the coldest part of the year because it takes time for the Earth to cool as we move through the winter season and it's coldest in late January/early February (and, similarly, not hottest at the June summer solstice but in late July/early August).
It is the shortest day of the year in the Northern Hemisphere. Here in the mid-Hudson Valley, we only have 9 hours and 39 minutes of time between sunrise and sunset. By New Year's Eve, we'll have 20 minutes more of daylight as the sun rises 8 minutes earlier and sets 12 minutes later.
I try to make it a point to be in tune with the sky. To notice the shortening and lengthening days, the movement of the Sun throughout the year, the changes in the constellations, and the phases of the Moon. The paper calendar we base our lives on is completely artificial but it's based, ultimately, on something real - the movements of the Earth and Moon in their orbits around a star. Something we can see and experience if we just look up and pay attention to the natural world around us.