The degrees and the days
Each sign has three faces of ten degrees. This is the middle one, which puts it well clear of the cusp at either end, so a placement here is rarely ambiguous about its sign even if it might be ambiguous about its decan.
The Sun works through it in about ten days, roughly 2 August to 12 August. The dates are approximate by design. Boundaries move by up to a day and a half from year to year, and someone born in the hours around a boundary can fall on either side depending on the year.
Mid August is also the part of the year when the Sun is furthest from any cardinal point, so nothing in the seasonal frame anchors these boundaries. They drift with the calendar exactly as the sign boundaries do.
Chaldean and triplicity agree here
The Chaldean sequence is Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, repeating from zero degrees of Aries through all thirty six faces. Counted out, the middle decan of Leo arrives at Jupiter.
The triplicity scheme assigns each decan to a sign sharing its element, in order. Fire runs Aries, Leo, Sagittarius, so the middle decan of a fire sign goes to Sagittarius, and Sagittarius belongs to Jupiter.
Both roads therefore end at Jupiter. This is unusual enough to state plainly, because most decans are contested and most published descriptions do not tell the reader which scheme they used. When a ruler is agreed, a reader can at least stop worrying about whose table they landed on.
What Sagittarius and Jupiter are said to add
The Sagittarius colouring is read as widening the frame. Leo on its own is described as concerned with a particular performance and a particular audience; with Sagittarius mixed in, the tradition describes the same appetite for display pointed at something larger, such as a cause, a body of knowledge or a place a long way from home. The interest is in the meaning of the thing rather than in the room it is done in.
Jupiter as face ruler is taken to add scale and a tendency to promise. Sources describe generosity that is genuine and commitments that outrun the calendar, so the same placement is credited with opening doors for other people and blamed for undertaking more than one person can carry out. Teaching, in the broad sense of explaining a subject to people who did not ask, comes up repeatedly in the older descriptions.
All of that is what the tradition reads into the combination, offered as interpretation. Jupiter's condition in a real chart, its house and its aspects, would change how much of it applies.
Traditional texts also use faces for timing and for judging strength, not only for description. A planet in its own face was counted as having a small amount of essential dignity, the weakest of the dignities but not nothing, which is part of why the faces were preserved at all.
Not only the Sun, and the boundary question
Every point in a chart falls in a decan. An Ascendant at fourteen degrees of Leo is in this face, and so is a Moon at nineteen degrees. The rising degree moves quickest, about a degree every four minutes, so decans do the most work there when separating charts of people born on the same day.
The technique deserves an honest description of its weight. Decans came into astrology from Egyptian star lists that marked the hours of the night, so they predate much of what surrounds them, but they function as a refinement rather than as a foundation. A chart reading that leaned on decans before sign, house and aspect would be built the wrong way up.
Because the boundary is a degree and not a date, the check above computes the actual position of the Sun for your birth date. For anyone born close to 2 August or 12 August that is the only way to get a definite answer.
Questions people ask about this
What dates are the second decan of Leo?
Roughly 2 August to 12 August. The boundaries shift by up to a day and a half between years, so a birth at either end should be checked against a computed solar degree.
Which planet rules the middle of Leo?
Jupiter, under both schemes. The Chaldean count reaches Jupiter on this face, and the triplicity order gives the middle fire decan to Sagittarius, whose ruler is Jupiter.
Does this decan dilute Leo?
The tradition describes it as redirecting rather than diluting. The Leo material stays, and the Sagittarius colouring is read as changing what it is aimed at, from a specific audience towards a broader subject.
What is the Chaldean order?
A fixed sequence of Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter, repeated through the thirty six decans from the start of Aries. It derives from the classical ordering of the planets by apparent speed.
Do decans apply to the Moon and Ascendant?
Yes. Any placement occupies a decan, and many practitioners read the Ascendant decan before the solar one because the rising degree changes fast enough to distinguish charts born hours apart.
Should I trust decan readings?
Treat them as a second-order refinement. They are an old and genuine part of the tradition, but sign, house and aspect carry far more of the reading, and the decan schemes themselves disagree on many faces.