What's happening
The Sun's ingress into Sagittarius happens when its ecliptic longitude crosses from 30° Scorpio into 0° Sagittarius, roughly a month before the winter solstice in the north. Sagittarius is the ninth sign of the tropical zodiac and the third mutable sign, closing autumn and pivoting toward winter at 0° Capricorn.
The Sun spends about thirty days in Sagittarius. In the northern hemisphere these are the shortest days of the year so far, the weeks leading into the solstice.
The tradition
Sagittarius is a mutable fire sign, ruled by Jupiter. The tradition describes it as the sign of the long view — travel, philosophy, religion, higher education, foreign cultures, the search for meaning, and the willingness to live for something larger than the day.
Where Scorpio was loyal to the private truth, Sagittarius is loyal to the larger story that makes a private truth worth telling. Jupiter's rulership gives the month a generous register; Sagittarius season is classically the most optimistic month of the year, and the one most likely to reward the bet you have been too cautious to make.
How to work with it
Pick the vision back up. Plan the trip. Apply for the program. Start the book — the one you are reading, or the one you are writing. Sagittarius season is well-suited to the conversation about what life is actually for, and to the first practical step toward an answer you can live with.
The shadow is confidence without patience — the sweeping plan that never meets a budget, the opinion that arrives without homework. Sagittarius at its best is curious; at its worst it is certain. The month rewards the traveller more than the lecturer.
The simple rule
Ask what you would aim at if you trusted you had time. The Sun in Sagittarius rewards the horizon you are willing to walk toward.