A decan that crosses the new year
Each sign holds three decans of ten degrees. This is the first in Capricorn, and its opening degree is the solstice point, where the Sun reaches its lowest declination in the northern hemisphere.
The Sun spends roughly ten days here, usually from about 22 December to about 1 January, which makes this one of the few decans that straddles the calendar year. The boundaries move by up to a day and a half between years, because the tropical year is not a whole number of days and the ingress drifts until a leap day pulls it back. Anyone born in the last days of December or the first days of January is close enough to an edge that the date alone will not settle it.
Saturn's ground, Jupiter's turn in the sequence
The triplicity method gives each decan of a sign to a sign of the same element in order. The earth signs run Taurus, Virgo, Capricorn, and a Capricorn sequence therefore begins with Capricorn itself, so this decan stays with Saturn, the sign's own ruler.
The Chaldean order takes no notice of any of that. It repeats Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter from the start of Aries across all thirty six decans, and the count happens to arrive at Jupiter here.
The disagreement is worth stating plainly, because Saturn and Jupiter are conventionally read as near opposites, one contracting and one expanding. An article calling these degrees the most Saturnine in the zodiac and another calling them the most generous part of Capricorn may both be reporting their scheme accurately without saying which one they used.
What each assignment is taken to mean
Capricorn on Capricorn is read as the sign showing without dilution. Nothing of Taurus or Virgo enters, so the emphasis falls on structure, long timescales, willingness to start at the bottom of something, and a habit of measuring the present against a target some distance off. Described unfavourably, it is treating a life as a schedule and being unable to stop working when the work is done.
Jupiter in the Chaldean assignment cuts against that. It is read as adding scope, patronage and a willingness to commit resources rather than conserve them. The older material on this face speaks of undertakings begun in earnest and of standing gained through others. Jupiter is in its fall in Capricorn, so the traditional reading is not of easy expansion but of ambition that has to be argued for and paid for, which is a narrower claim than simply calling the decan lucky.
Other placements, and the standing of decans
Any point falls in a decan, not only the Sun. An Ascendant at three degrees of Capricorn or a Moon at eight takes this one, and traditional practice treated the Ascendant's decan as at least as informative. The rulers set out above apply whatever the placement.
As for how much to make of it, decans descend from Egyptian star lists, groups of stars used to mark the hours of the night, and the rulership schemes were attached to them much later. That gives the divisions a long history, but the interpretive layer is younger and lighter than the sign material it sits inside. A decan refines a reading; it does not underwrite one. Sign, house and aspect do the structural work.
The check above computes your actual solar degree from your birth date, which is what a birthday in this particular window most needs, given that the decan spans the turn of the year and the sign boundary sits just behind it.
Questions people ask about this
What dates does the first decan of Capricorn cover?
Roughly 22 December to 1 January, so it crosses the new year. The boundaries move by up to a day and a half between years, which matters here because the earlier edge is also the Sagittarius boundary.
Is this decan ruled by Saturn or Jupiter?
Saturn under the triplicity scheme, since the decan stays with Capricorn itself, and Jupiter under the Chaldean order. Those planets are read very differently, so the scheme in use changes the description substantially.
Does the solstice make these degrees special?
Zero degrees of Capricorn is the December solstice point, which is an astronomical fact rather than an interpretation. The tradition treats the opening degrees of cardinal signs as marked, but that applies to all four, not to Capricorn alone.
What does Jupiter's fall in Capricorn mean here?
It means the tradition does not read the Chaldean ruler as bringing easy expansion. The usual reading is scope that has to be built and defended rather than given, which is a more limited claim than calling the decan fortunate.
Can decans be used for the Moon or Ascendant?
Yes, and they were, long before the Sun became the popular focus. Every placement occupies a ten degree division, and the Ascendant's decan drew particular attention in older practice.
What are decans, in short?
Ten degree divisions of a sign, three per sign and thirty six across the zodiac. They come from Egyptian star lists used for night timekeeping, and the planetary rulers described here belong to the later tradition.