Where it falls
Every sign is cut into three faces of ten degrees, thirty six of them around the whole ecliptic. This is the closing face of Leo, from twenty degrees to thirty.
The Sun spends roughly ten days crossing it, usually between about 13 August and about 23 August. That range should be treated loosely. Because the tropical year runs slightly longer than a whole number of days, every boundary slips by as much as a day and a half from one year to the next, and the exact hour of birth shifts it further. The upper edge here coincides with the ingress into Virgo, which is why the published Leo end date wanders between 22 and 23 August.
Both counts, one answer
The Chaldean order assigns rulers in the repeating sequence Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, beginning at zero degrees of Aries and continuing without interruption through the zodiac. Counted forward, the fifteenth face lands on Mars.
The triplicity scheme gets there another way. It gives the faces of a sign to the signs of its own element in order, so the fire decans of Leo belong to Leo, then Sagittarius, then Aries. Aries is ruled by Mars, so the third face is Mars again.
Agreement of this kind is uncommon. Across the wheel the schemes contradict each other far more often than they coincide, which is why a decan ruler quoted with no method named is usually unreliable. Here the question does not arise, and any source you consult should give you Mars.
What Leo with Aries in it is taken to be
The Aries colouring is read as Leo shorn of its patience. The sign's concern with being seen and with holding a position is assumed to remain, but this face is described as unwilling to wait for the position to be granted. Traditional writing associates it with taking the initiative in situations where authority is unclear, and with a preference for settling a matter quickly rather than managing it.
Mars arriving a second time by the Chaldean route is read as reinforcing the same tendency rather than adding a new one. Older texts treat a doubled ruler as a placement without a counterweight, so the qualities are described as showing plainly, for better and for worse. The heat is not moderated by anything within the decan itself, which means the rest of the chart carries the whole job of tempering it.
Note that this holds for any placement in the decan, not just a Sun. The Ascendant and the Moon are read through decans as a matter of routine. The system itself comes down from Egyptian star lists that were in use before the zodiac took its present shape, so it has real depth behind it, but it stays a second-order refinement rather than a foundation for a reading.
Settling a borderline birthday
Since a decan boundary is a degree rather than a date, someone born on 22 or 23 August may have a Sun in late Leo or early Virgo depending on the year and the time. The same uncertainty exists at the lower edge around 13 August, quieter but no less real.
The check above works out the Sun's actual longitude for your birth date and reports both the degree and the decan it falls in. That is the part a table of dates cannot do for anyone born close to a line.
Questions people ask about this
What dates make up the third decan of Leo?
Roughly 13 to 23 August, with the upper edge landing on the change into Virgo. Both boundaries move by up to a day and a half between years, so late August birthdays should be calculated rather than assumed.
Who rules the last decan of Leo?
Mars, under either method. The Chaldean sequence reaches Mars at this face, and the triplicity scheme assigns it to Aries, which Mars rules. It is one of the places where the schemes do not conflict.
Does the Aries face override the Sun sign?
No. The Sun remains in Leo, and the decan is read as a shading on that placement. It describes emphasis within the sign rather than a change of sign.
Is a doubled Mars considered aggressive?
Traditional texts describe it as undiluted rather than aggressive, which is a description of degree of expression, not of character. Whether it shows as decisiveness or as impatience depends on the aspects and the house.
Do decans apply beyond the Sun?
Yes. Every placement occupies one of the thirty six decans, and the Ascendant and the Moon are common places to apply the technique. The Sun is used here because it is the placement most readers know off by heart.
Where did decans come from?
From Egyptian star lists used to mark the hours of the night, later absorbed into Hellenistic astrology and matched with planetary rulers. That history is well documented and is the reason the system has survived.