system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If the winter solstice is the longest night of the year, why does it mark the beginning of winter, rather than the very middle of it?
gpt
The temperature of any given day has a bit to do with the sun, and a lot to do with the temperature of the land and sea.

The earth is constantly radiating heat, and the shortest day is when it will absorb the least heat from the sun - but it still has a lot of heat left over from autumn.

Over the next couple of months, although the energy being absorbed from the sun is more than the day before, it is still less than is given off each day. So every day, the land and sea (and thus the climate of that hemisphere) gets colder and more wintery.

The balance doesn't shift until the hemisphere is absorbing more energy each day from sunlight than it is radiating, which is around the time that day and night are equal lengths (i.e. the vernal equinox).

So, the coldest months are between the winter solstice and the vernal equinox.