The degrees and the ten days
Twenty to thirty degrees of Scorpio, the last of the sign's three ten degree divisions. The Sun crosses it in roughly ten days, ordinarily from about 13 November to about 21 November, with the far end running into the change of sign.
Treat both dates as approximate. The tropical year runs slightly over three hundred and sixty five days, so ingresses slide through the calendar and any boundary can sit up to a day and a half from where it fell the year before. That is particularly worth knowing at the closing edge, since a birth on 21 or 22 November may be late Scorpio or early Sagittarius depending on the year and the hour.
Venus here, or the Moon by another route
The Chaldean order cycles Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter from the start of Aries and continues without interruption through all thirty six decans. Counted out, the closing decan of Scorpio falls on Venus.
Triplicity works by element instead. The water signs run Cancer, Scorpio, Pisces, so a Scorpio decan sequence goes Scorpio, Pisces, Cancer, and this third one belongs to Cancer, and through Cancer to the Moon.
Venus and the Moon are not opposites in the way Mars and Venus are, but they are describing different things, and the disagreement is real. Most articles on Scorpio decans quote one ruler and leave the method unstated, which is how the same band of degrees turns up described as Venusian in one place and lunar in another. Naming the scheme costs a clause and removes the confusion.
What each is read as contributing
Scorpio tinted by Cancer is read as the sign's intensity turned towards attachment rather than investigation. The tradition attaches to these degrees a long memory for how people have behaved, protectiveness towards a small number of them, and a reluctance to let a matter be closed by anyone else's timetable. The same trait described unfavourably is holding a grievance long after the occasion has passed.
Venus, in the Chaldean assignment, is taken to add attraction and value to that. The older texts on this face speak of pleasure, of things wanted rather than needed, and of arrangements between people that are not purely practical. Placed in Scorpio, where Venus has no dignity, it is usually read as desire that is specific and undivided rather than easy or diffuse. Whether that reads as devotion or as fixation is exactly the kind of judgement the tradition leaves to the rest of the chart.
Beyond the Sun, and how far to take it
Decans are not tied to any one placement. An Ascendant at twenty seven degrees of Scorpio takes this decan, as does a Moon at twenty two, and older practice read the Ascendant's decan with as much interest as the solar one. Whatever occupies the degrees takes the rulers described here.
Some perspective on the technique. Decans entered astrology from Egyptian star lists, groups of stars used to mark the hours of the night, and the planetary rulers were fitted to them long afterwards. The lineage is genuine and old, older in some respects than parts of the material built on top of it. That still does not make it foundational. A reading rests on sign, house and aspect, and the decan adjusts the tone. It is a refinement, and reading it as more than that overstates what ten degrees can carry.
The check above works out the true solar degree for your birth date and reports which decan holds it. For a birth in the closing days of Scorpio that is the difference between an answer and a guess.
Questions people ask about this
What dates are the third decan of Scorpio?
Roughly 13 to 21 November, with movement of up to a day and a half between years. The later edge runs into the Sagittarius boundary, so births there should be checked by degree rather than by date.
Is Venus really the ruler of a Scorpio decan?
In the Chaldean order, yes. The sequence takes no account of which planets suit which signs, so Venus falls in these degrees despite having no dignity in Scorpio. The triplicity scheme gives the same decan to Cancer and the Moon.
Why does Cancer appear in a Scorpio decan?
Because triplicity allots a sign's decans to the signs of its own element in order. For Scorpio that order is Scorpio, Pisces, Cancer, so the final decan goes to Cancer.
Does this decan make someone less intense?
The tradition reads the Cancer tint as redirecting the intensity towards attachment rather than reducing it. That is an interpretation of the blend, not a measured difference, and the Chaldean reading would put it another way again.
Do decans work for the Ascendant and Moon?
Yes. Every point in a chart falls in a ten degree division, and the Ascendant's decan was a standard part of older practice. The Sun is emphasised here only because it is the placement most readers arrive with.
How seriously should decans be taken?
As a refinement rather than a foundation. They are an old and real part of the tradition, descended from Egyptian star lists, but they shade a reading that sign, house and aspect have already set.