Where it sits in the year
Zero degrees of Cancer is one of the four cardinal points of the tropical zodiac, the moment of the June solstice. This decan runs from there to ten degrees, so it opens the whole water quarter of the year.
The Sun takes about ten days to cross it, from around 21 June to around 1 July. Those dates are not fixed. The solstice itself moves within a window of about a day and a half across the leap cycle, and the far boundary of the decan moves with it.
Where the schemes disagree
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and repeats from zero degrees of Aries. Counting through the faces of Aries, Taurus and Gemini brings the sequence round so that the first face of Cancer falls to Venus.
The triplicity scheme takes no notice of that count. It gives each decan to a sign of the same element in order, and the water signs run Cancer, Scorpio, Pisces. The opening decan of Cancer therefore goes to Cancer itself, which the Moon rules.
So a reader will find this decan described as Venus ruled in one source and Moon ruled in another, with neither naming its method. Both are given here for that reason. Where the schemes conflict, the sensible response is to treat neither as settled and to read the sign and the chart first.
It is worth adding that the Chaldean order is not arbitrary. It follows the classical arrangement of the planets by apparent speed, from Saturn as slowest to the Moon as fastest, run backwards and repeated. The triplicity assignment has no such derivation; it is simply an element counted in order.
The two readings on offer
By triplicity this is Cancer doubled, with nothing borrowed from Scorpio or Pisces. The tradition reads that as the sign's habits shown without complication: attachment to a particular place and to a small number of people, memory that holds detail long after it was useful, and a defensive reaction that arrives before any assessment of whether defence was needed.
The Chaldean face ruler pulls in a different direction. Venus is taken to add a concern with keeping relations pleasant, so the same protectiveness is described as expressed through hospitality and smoothing rather than through withdrawal. The tension the sources describe is between wanting the surface calm and wanting the truth said, and the stated cost is a difficulty raising a grievance until it has accumulated.
These are interpretations, and the schemes disagreeing means there is no single traditional line to report here.
Any placement, and the boundary problem
Decans are not a solar technique. A Moon at four degrees of Cancer is in this face, as is an Ascendant at nine degrees, and the Moon in Cancer is already a placement most systems treat as significant before any decan is considered. The Ascendant is where decan divisions cut most finely, because it advances a degree roughly every four minutes.
It is worth keeping the scale of the technique clear. The decans are old, descending from Egyptian star lists used to mark night hours, and they were absorbed into astrology rather than invented by it. Even so, they are a refinement on top of sign, house and aspect, not a replacement for any of them.
The check above computes your solar degree directly, which is the only way to resolve a birth on or near 21 June or 1 July. A date range cannot do it, because the ranges themselves move.
A further caution applies to the opening degrees of any cardinal sign. Because zero degrees of Cancer is the solstice itself, a birth in the hours around it can fall in late Gemini, and no date table will show that.
Questions people ask about this
What dates fall in the first decan of Cancer?
Around 21 June to 1 July, starting at the solstice. Both ends drift by up to a day and a half across the leap cycle, so a computed degree is the reliable answer near the edges.
Is this decan ruled by Venus or the Moon?
Venus by the Chaldean order, the Moon by the triplicity scheme. The disagreement is real and not a mistake in either source. Which one you use is a choice of method, so it is worth knowing which your source made.
Why do sources contradict each other on decan rulers?
Because two different assignment systems are in circulation and most writing quotes one without naming it. Once you know which method a source used, the contradiction usually resolves into a straightforward difference of scheme.
Does the first decan make someone more Cancerian?
That is the traditional claim under the triplicity scheme, since nothing from Scorpio or Pisces is mixed in. It is an interpretation rather than a measurable effect, and the Chaldean scheme reads the same degrees differently.
Can decans be used for the Moon?
Yes. Every placement sits in a decan, and the technique is applied to the Moon and the Ascendant as readily as to the Sun. This page centres the Sun because that is the usual starting question.
How old is the decan system?
The divisions come from Egyptian star lists, where thirty six star groups marked hours through the night. Planetary rulership was attached later, in the Hellenistic period, which is when the Chaldean order enters the picture.