Position in the sign and in the calendar
Signs divide into three ten degree parts, and this is the last of them in Cancer, running from twenty degrees to thirty. Its close is also the close of the sign.
The Sun crosses these degrees over roughly ten days, usually from about 13 July to about 22 July. Those figures shift a little each year. The tropical year is not a whole number of days, so every ingress moves by up to a day and a half, and a birthday on 22 July may be a late Cancer degree in one year and a Leo one in the next.
The Moon by one count, Pisces by the other
The Chaldean order steps through Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and repeats, starting at zero degrees of Aries and running unbroken across all thirty six decans. Counted out, the final decan of Cancer lands on the Moon.
The triplicity scheme takes a different route. Each decan of a sign goes to a sign of the same element, in order, so the water decans run Cancer, Scorpio, Pisces. The third decan of Cancer therefore belongs to Pisces, which in modern practice is associated with Neptune and in older practice with Jupiter.
Unusually, the two do not really fight here. The Moon rules Cancer, so the Chaldean answer reinforces the sign, and the Pisces assignment softens rather than opposes it. Where the schemes do matter is in the detail of what is being claimed, and in the fact that an article naming just one ruler is still telling you only half of what is in circulation.
What is read into the closing degrees
Cancer shaded by Pisces is traditionally read as the sign's attentiveness to other people losing some of its defensiveness. Where the opening decan is described in terms of guarding a small circle, this one is described in terms of boundaries that are harder to locate, an interest that extends past the household, and a response to atmosphere rather than to stated facts. Read from the other side, that is difficulty in knowing which feelings in a room are one's own.
The Moon in the Chaldean assignment is taken to add change of state: cycles, tides, moods that turn over quickly and a memory that holds onto detail. Older texts on this face lean on movement over water and on things that come and go on a schedule. Since the Moon also rules the sign, the usual reading is that the lunar character is stated twice rather than balanced by something else.
Any placement, and the limits of the technique
These divisions apply to whatever sits in them. An Ascendant at twenty five degrees of Cancer takes this decan, and so does a Moon at twenty eight. Traditional practice read the Ascendant decan closely, and the Sun became the popular focus only much later.
It is worth being clear about the standing of the technique. Decans descend from Egyptian star lists, groups of stars used to mark the hours of the night, and only later acquired the planetary rulers set out above. That gives them a long history, but a long history is not the same as a load-bearing role. Decans qualify a reading that already rests on sign, house and aspect, and they are best treated as a refinement rather than a starting point.
Because the boundary at thirty degrees is also a sign boundary, births in the last days of Cancer are the ones most often misfiled. The check above computes the real solar degree from your birth date and reports which side of the line it falls on.
Questions people ask about this
What dates does the third decan of Cancer cover?
About 13 to 22 July, though the edges move by up to a day and a half between years. The later boundary is also the boundary with Leo, so a birthday there is worth checking properly.
Who rules the final decan of Cancer?
The Moon under the Chaldean order. The triplicity scheme instead assigns it to Pisces, and so to Neptune in modern practice or Jupiter in older practice. Both answers appear in print.
Why does Pisces come into a Cancer decan?
Because triplicity distributes each sign's decans among the signs of its own element in order. Cancer, Scorpio and Pisces are the water signs, so the last water decan of Cancer falls to Pisces.
Is this decan the same as being on the Cancer to Leo cusp?
No, although they overlap at one end. The decan is a fixed ten degree band, while cusp talk usually refers to a vague window around the sign change. The degree itself is what settles the sign.
Do decans apply to the Moon and Ascendant?
Yes. Every placement sits in a decan, and the Ascendant and Moon were read this way long before the Sun became the usual focus. The rulers above apply to whichever point falls in these degrees.
How old is the decan system?
The divisions come originally from Egyptian star lists used for timekeeping at night, which predate the planetary rulerships attached to them. The interpretations above belong to the later tradition and are interpretation, not measurement.