What's happening
At 0° Capricorn, the Sun reaches its southernmost declination. In the northern hemisphere this is the winter solstice — the shortest day of the year — and in the southern hemisphere it is the longest. For a few days around the exact moment the Sun appears to stand still in declination before starting back the other way. That stillness is where the word solstice comes from.
The Sun remains in Capricorn for about thirty days before moving on to Aquarius.
The tradition
Capricorn is the cardinal earth sign, ruled by Saturn. Cardinal signs begin seasons; Capricorn begins winter, the half of the year when warmth has to be earned and stored. The tradition reads this as the season of structure, accountability, long projects, and the slow climb associated with Saturn's rulership.
The solstice moment itself has been treated as a turning point in most cultures that kept a calendar. The light does not immediately return, but the direction of travel has changed. That is the symbolic grammar of the Sun in Capricorn: commitment at the darkest point, on the faith that the year is turning.
How to work with it
This is weather for plans that have to outlast enthusiasm. Set the calendar, accept the limits, and choose the few priorities you are willing to carry through the cold part of the year. Capricorn trusts repetition over intensity.
The shadow is mistaking endurance for meaning. Do the work that compounds. Stop doing the work that only hurts.
The simple rule
Ask what is worth doing in January that nobody will notice until June. The Sun in Capricorn builds by increments, and it keeps what it builds.