system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 Why does the new year lag behind the winter solstice by 10 days?
gpt
For most of the ancient world the new year began with the arrival of spring. For the Romans this was March...although the March 20/21 equinox is considered the "first day of spring" this is more of an astronomical event and not a definitive herald of when "spring" actually begins. To the peoples of the ancient world "spring" began when it got warmer, and that was a few weeks earlier than the equinox.

So the first Roman calendar ran from March to December, and then they just tacked on "winter" at the end without really even consistently counting how long it was. When this began to get silly they finally added in two extra months for winter, January and February. Around the 2nd century BC, toward the end of the Roman Republic, they moved the new year from the beginning of March to the beginning of January for short-term political reasons. It's stayed there ever since, at least in the Western world.