22 November to 21 December
Sagittarius spans two hundred and forty to two hundred and seventy degrees of ecliptic longitude, and its closing boundary is the December solstice, the Sun at its furthest south. That gives the end of this season a definition independent of any astrological source: it is the shortest day in the northern hemisphere.
The opening crossing lands on 21 or 22 November and the closing one on 21 or 22 December. Both alternate on the leap cycle, because the calendar rounds a tropical year to whole days and repays the difference every fourth February.
Mutable fire, closing a quarter of the year
The mutable signs end the seasons, and Sagittarius ends the autumn quarter. Through the month the Sun's decline slows as it approaches the solstice, so the change in day length shrinks from day to day even as the days themselves are shortest.
Jupiter rules the sign and Pisces as well. Sagittarius has no traditional exaltation, and its decans fall to Mercury, the Moon and Saturn. The galactic centre currently lies near twenty seven degrees of the sign, which is a modern observation rather than part of the classical tradition, and one worth labelling as such.
The sidereal version of this range
Charts cast in India put most of these births in Scorpio. The reason is a difference of anchoring: the tropical zodiac counts from the equinox point, the sidereal one from the fixed stars, and precession has pulled those two apart by roughly twenty four degrees since they last agreed.
That gap is a little over three weeks of the Sun's motion, so sidereal Sagittarius runs from about the middle of December into the middle of January. Neither system is a corrupted copy of the other.
What a date range does and does not settle
It settles the Sun, and only when the birth is several days clear of both edges. Nearer than that, the range is unusable, since it was rounded to whole days at some point and the crossings keep moving underneath it. The year, the time and the place give an exact longitude and therefore one sign.
Everything else in the chart is out of reach of a calendar square. The Moon changes sign every two or three days and the Ascendant depends on the hour and the latitude of the birth.
Questions people ask about this
What are the Sagittarius dates?
Approximately 22 November to 21 December. The exact start falls on 21 or 22 November and the end on 21 or 22 December, varying with the year and with the time zone the birth is dated in.
Why do the dates change from year to year?
Because the tropical year is not a whole number of days. The ingress slips about six hours later each year and is pulled back by each leap day, so it oscillates across roughly a day and a half.
When is Sagittarius season this year?
Within the range above, at a minute that has to be calculated for the specific year. Anyone born close to either edge needs the actual solar position rather than a table.
Why does Vedic astrology not make me Sagittarius?
It measures from the fixed stars rather than from the equinox, and the two references have separated by about twenty four degrees. That moves every sidereal boundary later by a little over three weeks.
Does being born on the first day of Sagittarius mean anything special?
It puts the Sun in the first degree of the sign, which matters for aspects and house placement. The tradition does not read it as a weaker or stronger version of the sign itself.