The only boundary between two lights
At a hundred and twenty degrees of ecliptic longitude the Moon's sign gives way to the Sun's. It is the one changeover on the wheel where both rulers are luminaries rather than planets, and the pair are the two bodies whose motion defines day and month in the first place. Cardinal water becomes fixed fire at the same stroke.
The crossing falls on 22 or 23 July. Which of the two depends on the year within the leap cycle, because the Sun's circuit is about a quarter of a day longer than the calendar allows for and the accumulated error is wiped out every fourth February.
There is no partly-both
Solar longitude is one number at one moment. Before the mark the Sun is entirely in Cancer, after it entirely in Leo, and the interval between those states has no duration. Neither the older texts nor contemporary technique defines an in-between placement, and none of the traditional dignities, aspects or decans would know what to do with one.
Where the term cusp actually applies
Houses have cusps. They are boundaries derived from the horizon and meridian of the birthplace, they rotate through the whole zodiac in a day, and they differ between two people born at the same instant in different cities. Signs are fixed thirty degree arcs and share none of those properties.
Folklore has attached its own weight to this stretch of July, which overlaps the old dog days named for the heliacal rising of Sirius. That is a calendar tradition about a star, not a statement about the Sun changing sign.
Each side on its own terms
The Sun in late Cancer is read exactly as it is in early Cancer: the Moon's sign, protective, tidal, slow to open. The closing decan of Cancer is the Moon's own in the Chaldean order, the most concentratedly lunar band of the whole zodiac, which is the opposite of a fading influence.
Leo then opens under Saturn in that same scheme, giving the first degrees of the Sun's own sign a rigidity that later Leo degrees lose. Neither of those effects has anything to do with proximity to the line.
Working out a 22 or 23 July birth
The year, the clock time and the place produce a longitude, and the longitude answers the question outright. Below a certain closeness to the mark this is the only method that works, because printed ranges were rounded to whole days and disagree with one another.
A birth several days clear of the boundary needs none of this. Inside about forty eight hours, nothing except the calculation will do.
Questions people ask about this
Am I a Cancer Leo cusp?
No, because that placement does not exist. The Sun was either short of a hundred and twenty degrees at your birth or past it, which makes you Cancer or Leo.
What date does the Sun move from Cancer to Leo?
22 or 23 July, depending on the year and on the time zone the birth is dated in. The instant is the same worldwide; only the local calendar date differs.
Do I have traits of both signs?
Popular astrology says so and sells the blend as a type. The chart does not show it: one Sun, one longitude, one sign. Other placements can of course sit in either sign.
What is a cusp in real astrology?
A house boundary. Houses are divided using the horizon and the meridian of the birthplace, and their cusps are genuine dividing lines within a chart.
How close to the boundary do I need to be to bother checking?
About forty eight hours either side. Inside that window the year and the birth time decide the answer; outside it, any date range gives the same result.