The two boundaries
Scorpio begins when the Sun reaches 210 degrees of ecliptic longitude and ends when it reaches 240. Unlike Libra or Capricorn, neither of those points coincides with an equinox or a solstice. Scorpio sits squarely in the middle of the northern autumn, halfway between the September equinox and the December solstice, which is exactly what a fixed sign is for in the seasonal logic of the tropical zodiac: it holds the middle of a season rather than starting or ending one.
Because there is no named astronomical event to anchor it, the popular date range for Scorpio is a little vaguer than for the cardinal signs. Lists print 23 October to 21 November, or 24 October to 22 November, and both are defensible averages. The table above avoids the averaging entirely by solving each boundary for the year in question.
Why the range shifts
The mechanism is the calendar, not the sky. Earth takes about 365.2422 days to complete an orbit, so each common year of 365 days leaves the Sun about six hours short of where it was on the same date twelve months earlier. Sign boundaries therefore creep later on the clock through a four year cycle, then jump back when February gains a day.
Across a decade a given boundary moves through a window of roughly a day and a half, which is enough to change the printed date. That is why the boundary you were told as a child may not match the boundary that applied in your own birth year.
A sign on the shortening side of the year
Scorpio runs close to thirty days, and it is measurably shorter than the summer signs. Earth reaches perihelion, its closest approach to the Sun, in the first week of January, and Kepler's second law means the planet travels fastest when it is nearest. From October onwards the Earth is picking up speed, so the Sun appears to cross each thirty degree arc a little more quickly than the one before.
Cancer and Gemini, crossed near aphelion in July, run to about 31.5 days. Capricorn, Sagittarius and Aquarius are the short ones at roughly 29.5. Scorpio sits on the descending slope between the two extremes, which is why its length is neither remarkable in itself nor quite equal to Taurus opposite it.
Ruler, element and the modern usage
Scorpio is fixed water. Its traditional ruler is Mars, and since the discovery of Pluto in 1930 most modern practice assigns it Pluto instead, usually keeping Mars as a co-ruler. Reading the sign through Mars gives you endurance and force of will, reading it through Pluto gives you the material about buried things and irreversible change. Neither reading is complete on its own.
The phrase Scorpio season itself is newer than any of that. It came out of astrology on social media over the last decade and now simply denotes the month the Sun spends in the sign. It applies to everyone at once and is not a claim about anybody's own chart, though where it lands for you depends on which house Scorpio occupies in your natal wheel.
Questions people ask about this
Is Scorpio season 23 October to 21 November?
That is a good average and a poor answer for anyone born near either edge. Both boundaries move within a window of about a day and a half across the leap year cycle. Pick your year in the table above and you get the ingress to the minute in UTC.
Who rules Scorpio, Mars or Pluto?
Both, depending on the tradition. Mars is the classical ruler and was the only one until Pluto was found in 1930. Most modern astrologers use Pluto as ruler and keep Mars as co-ruler, and reading a Scorpio placement through both usually gives more than either alone.
Why is Scorpio not exactly thirty days?
Because the zodiac divides the ecliptic into equal arcs but the Earth does not travel at an equal speed. The orbit is an ellipse, so the planet moves fastest near perihelion in January and slowest near aphelion in July. Autumn signs are crossed while the Earth is speeding up, making them slightly shorter than the summer ones.
Does Scorpio season relate to Halloween or the Day of the Dead?
Only by coincidence of date, though the association is old. Those festivals fall near the cross quarter point between the September equinox and the December solstice, which is roughly the midpoint of Scorpio. The seasonal placement is real; any further connection is interpretation rather than astronomy.
Am I a Scorpio if I was born on 22 November?
Possibly, and possibly a Sagittarius. The boundary can fall on the 21st or the 22nd and at any hour of the day. Your birth time and the time zone in force at your birthplace settle it, and a date range never can.