A hundred and fifty degrees, on 22 or 23 August
The Sun leaves its own sign at a hundred and fifty degrees of ecliptic longitude and enters Mercury's earth sign. In current years that instant falls on 22 or 23 August, timed to the minute rather than to a date.
A hundred and fifty degrees is also the quincunx, the aspect the tradition treats as the hardest to reconcile because the two signs involved share element, mode and polarity with each other in none of the usual ways. That the season boundary and the aspect angle coincide here is a property of the number, not a claim about anyone's character.
Fixed fire to mutable earth
The Sun rules Leo and rules nothing else; Mercury rules Virgo and Gemini both, and is exalted in Virgo besides, at fifteen degrees. So the handover is from a planet with a single domain to the one planet with a double claim on its own sign.
That is a real structural asymmetry, and it is more interesting than the blended type sold for these dates. The blend does not exist. A single ecliptic longitude cannot occupy two of the twelve equal divisions at once.
Cusps belong to houses
The technical sense of the word describes the boundary between houses, which are carved out of the local sky using the horizon and the meridian. Change the birthplace by a few hundred miles and every house cusp moves; the sign boundaries do not move at all.
What is genuinely true about births near this date is narrower: a range printed on a chart or a poster cannot settle them, because the range was rounded to whole days and the crossing keeps shifting inside them.
Late Leo and early Virgo
The final decan of Leo goes to Mars in the Chaldean scheme, the most combative third of a sign already read as proud, and the closing degrees of Leo are described in the texts with no reference whatever to the sign that follows.
Virgo then opens under the Sun in that same order, which is why the earliest Virgo degrees are read as more confident and less self-effacing than the rest of the sign. Both of those are decan facts with named rulers, and both are far more specific than any story about a boundary.
Deciding a particular birth
Supply the year, the time and the town, and an ephemeris returns the Sun's position to a fraction of a degree. It falls short of a hundred and fifty degrees or beyond it, and that is the whole answer.
Where the calculation contradicts a lifetime of assuming one sign, only the solar placement changes. Every other body in the chart was computed from the same data and is unaffected.
Questions people ask about this
Am I a Leo Virgo cusp?
No, because that placement does not exist. The Sun was either short of a hundred and fifty degrees at your birth or past it, which makes you Leo or Virgo.
What date does the Sun move from Leo to Virgo?
22 or 23 August, 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.