The Sun in its own sign
In traditional rulership each planet governs two signs, one on the solar side of the zodiac and one on the lunar side, except for the two lights themselves, which take one each. The Moon has Cancer and the Sun has Leo. This makes Leo the only month in which the Sun is, in astrological terms, at home.
The consequence for interpretation is that a Leo placement is read against the Sun's own condition in the chart rather than against a separate ruling planet's position. It also concentrates the traditional themes of the sign, visibility, authority, performance and warmth, around solar imagery rather than borrowing from anywhere else. Whether the reading holds up is a matter of belief. The transit itself is a measurable fact and the table above is where it is stated.
The dates and why they slide
The usual range printed is 23 July to 22 August. In practice the entry into Leo has been landing on the 22nd of July more often than the 23rd in recent decades, and it will keep creeping earlier until the leap year pattern nudges it back. This happens because the tropical year runs about 365.2422 days against a calendar of 365 or 366, so every boundary in the zodiac slips roughly six hours later per ordinary year and then jumps back a day when February gains one.
Nothing about the sky is irregular here. The irregularity is entirely in the calendar, which has to approximate a fraction with whole days. Solving for a specific year removes the approximation, which is why the table gives times rather than a range.
Length, and the position in the year
Leo follows Cancer, which contains the aphelion in early July, so the Sun is still moving slowly when it crosses this arc. Leo therefore takes about 31.4 days, close to the maximum. From here the Earth accelerates gradually as it swings back towards perihelion in January, so each subsequent sign is fractionally shorter than the last until Capricorn.
Leo is also a fixed sign, meaning it occupies the settled middle of its season rather than the beginning or the end. It sits between the June solstice and the September equinox with no astronomical landmark at either boundary, which is the ordinary case for eight of the twelve signs. Its element is fire, the third of the four applied to it after Aries and before Sagittarius, and fixed fire is the combination that produces the sign's reputation for sustained rather than impulsive effort.
Late July and late August birthdays
If your birthday falls on 22 or 23 July, or 22 or 23 August, the range you were given as a child may well have assigned you the wrong sign. There is no cusp between Cancer and Leo, or between Leo and Virgo. The Sun has one longitude at any instant and it lies inside one sign, so the question always has a definite answer once the birth time and the correct historical time zone are supplied.
That last point catches people out more than anything else. Summer time rules changed repeatedly through the twentieth century, and several countries shifted their standard offset entirely. Applying today's rule to a birth in 1948 will occasionally move the calculation by an hour, which is enough to change the answer for a birth a few minutes either side of a boundary.
For readers south of the equator, the dates need no adjustment. Leo season is the same month everywhere, but it falls in the depth of winter rather than the height of summer, and the solar imagery attached to the sign reads rather differently from there.
Questions people ask about this
Is Leo season 23 July to 22 August?
Roughly, and that is the range most references print. The actual entry has more often fallen on 22 July in recent years, and both boundaries move by up to a day across the leap year cycle. The table above gives the real times for your chosen year.
Why does the Sun rule Leo?
In traditional rulership the two lights take one sign each, the Moon in Cancer and the Sun in Leo, while the other planets take two apiece. The scheme is arranged symmetrically around those two, which is why Leo and Cancer sit next to each other.
How long does Leo season last?
About 31.4 days. The Sun is still travelling slowly after aphelion in early July, so Leo is one of the longer signs, well above the 29.5 days of the signs crossed near January's perihelion.
Is Leo a summer sign?
In the northern hemisphere it covers the middle of summer, which is where its imagery comes from. In the southern hemisphere the identical dates fall in midwinter. The ingress times are the same for everyone.
Where does the phrase Leo season come from?
It is a recent way of speaking, popularised through astrology on social media over roughly the last decade, where a sign became a convenient name for the month the Sun spends in it. The underlying period is ancient and astrologers previously called it the Sun's transit through Leo.