The degrees and the dates
This decan runs from ten degrees of Gemini to twenty degrees, the second of the sign's three ten degree divisions. The Sun crosses it in about ten days, ordinarily somewhere between 1 June and 11 June.
Take those dates as a guide rather than a rule. Because the tropical year runs a little over three hundred and sixty five days, the whole calendar of sign and decan ingresses slides by up to a day and a half from year to year. Anyone born on the first or the eleventh of June needs the actual degree, not the range.
Mars against Libra
The Chaldean order is a plain repeating sequence: Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, begun at zero degrees of Aries and carried through all thirty six decans without restarting at each sign. Counted through, the middle decan of Gemini falls on Mars.
The triplicity scheme allots the decans of each sign to signs of the same element in order. For the air signs that order is Gemini, Libra, Aquarius, so the second decan of Gemini is given to Libra, and through Libra to Venus.
These are not close readings of one another. Mars and Venus are traditionally treated as near opposites in temperament, so an article describing this decan as combative and another describing it as conciliatory may both be reporting their scheme faithfully. What is worth objecting to is an article that gives one ruler and does not say which count produced it.
The Libra tint and the Mars overlay
Under triplicity, Gemini shaded by Libra is read as the sign's talkativeness turned outward towards other people. The interest is less in information for its own sake and more in comparison, in weighing one account against another and in the effect a sentence has on the person hearing it. Traditional descriptions of this stretch lean on exchange, agreement and the negotiating that sits between them.
Mars, in the Chaldean assignment, is read as putting an edge on that. The older texts on the faces of Gemini in these degrees speak of argument, of pressing a point, and of activity undertaken quickly rather than deliberated over. Where Libra suggests weighing, Mars suggests deciding and moving. Combined, the usual reading is a mind that debates rather than merely discusses, though nothing here says whether that lands well or badly in a given life.
Other placements, and what decans are worth
Nothing about this is confined to the Sun. The Ascendant falls in a decan, so does the Moon, and in traditional practice the decan of the Ascendant carried at least as much interest as the solar one. If your Moon sits in these degrees the same rulers apply to it.
Some honesty about the technique is in order. Decans have real antiquity behind them: they began as Egyptian star groups used to tell the hours at night, and were absorbed into astrology proper much later, which is when the rulership schemes were laid over them. But antiquity is not the same as importance. Decans sharpen a reading that already rests on the sign, the houses and the aspects. They are a refinement, and treating them as a foundation puts more weight on ten degrees than the tradition itself does.
The check above works out your solar degree from your date and reports both rulers, which is the only way to settle a birth close to a boundary.
Questions people ask about this
When is the second decan of Gemini?
Roughly 1 June to 11 June in most years. The boundary can move by around a day and a half either way, so a birthday at the edge of that window should be checked against the actual solar degree.
Which planet rules the middle decan of Gemini?
Mars if you follow the Chaldean order, Venus if you follow the triplicity scheme through Libra. The gap between those answers is wider here than in most decans, which is why the scheme has to be named.
Why is Libra involved in a Gemini decan?
Because the triplicity method hands each decan to a sign of the same element in sequence. Gemini, Libra and Aquarius are the air signs, so the middle air decan of Gemini goes to Libra.
Do decans matter for the Ascendant?
Yes, and historically the Ascendant decan drew as much attention as the solar one. Every point in a chart sits somewhere within a ten degree division, so the same rulers can be read against the Moon or a planet.
Where do decans come from?
From Egyptian star lists that divided the night into hours by which groups of stars were rising. They were later matched to the thirty six ten degree divisions of the zodiac and given planetary rulers.
How much should I read into a decan?
Less than into the sign itself. Decan work is a genuine part of the tradition but a second-order one, meant to shade a reading rather than carry it. It is interpretation, and this page marks it as such.