What's happening
The Sun's ingress into Scorpio happens when its ecliptic longitude crosses from 30° Libra into 0° Scorpio, roughly a month after the autumnal equinox. Scorpio is the eighth sign of the tropical zodiac and the third of the fixed signs. The Sun spends about thirty days there, during which the days in the northern hemisphere continue to shorten and the year enters its darker half.
The tradition
Scorpio is a fixed water sign, ruled by Mars in classical astrology and co-ruled by Pluto in modern. The tradition associates it with depth, trust, loyalty, desire, fear, grief, power, and the kind of truth that cannot be told until the room is private.
Where Libra concerned itself with the surface of relationship — proportion, manners, agreement — Scorpio is concerned with what lives under that surface. The tradition reads the month as the annual check-in with what has been avoided: a debt, a resentment, a loss, a love, an appetite. Scorpio season is not gentle, but it is honest in a way the rest of the year often is not.
This is also the season of many cultures' festivals of the dead — Samhain, Día de los Muertos, All Souls — and the tradition treats that cultural rhyme as part of the sign's weather.
How to work with it
Tell the truth you have been rehearsing. Look at the finances, the medical thing, the relationship, the habit. Let whatever has been underground come up for air. Scorpio season rewards the courage to stop pretending and punishes the energy it takes to keep pretending well.
The shadow is suspicion mistaken for insight, or control mistaken for intimacy. Scorpio at its best is depth with a spine; at its worst it is secrecy that confuses privacy with power.
The simple rule
Ask what has been waiting to be said. The Sun in Scorpio rewards the conversation you have been postponing since spring.