Near the edge without being on it
Virgo begins at thirty degrees of ecliptic longitude counted from Leo's start, and on this date the Sun has never reached more than about twenty eight point nine at noon. That leaves at least a degree of clearance, which is more than a full day of solar motion.
A birth late on the twenty first in one of the higher years narrows the gap, since the Sun keeps advancing through the evening, but it does not close it. The genuine uncertainty starts on the days that follow, where the ingress itself falls and where the year and the hour and the time zone all begin to matter. On the twenty first they do not.
Why proximity is not contamination
People with birthdays in the last days of a sign are often told they carry something of the next one. There is no mechanism for that. The Sun occupies a single longitude at any instant, and twenty eight degrees of Leo is inside Leo in exactly the way that eight degrees is.
What is true is that a position near a boundary forms aspects to things in the early degrees of other signs, and that it may sit close to a house boundary depending on the chart. Those are consequences of the number rather than of the sign, and they are why the degree is worth knowing on its own account.
The band that is closing
Twenty eight degrees is deep in the third decan of Leo, which runs from twenty degrees to the end of the sign. Mars rules it under the Chaldean count, arrived at by ordering the planets by apparent speed from the first degree of Aries, and under the triplicity scheme as well, arrived at through Aries as the opening fire sign.
The tradition reads Leo as fixed fire under the Sun, and reads this final band as its most assertive part. Those readings come from the written record and are best used with that provenance attached. The longitude is a different sort of statement: it is checkable, and every ephemeris agrees on it.
The band edge behind this position, at twenty degrees, was crossed more than a week earlier, and the sign edge ahead has not been reached. That leaves the twenty first in an unusual spot: near a boundary in the ordinary sense of the word, yet with nothing about the birth that any boundary makes uncertain. It is the last date in Leo of which that can be said.
What remains open on a settled date
Settling the sign and the band settles the two coarsest facts available. The Moon moves twelve to fifteen degrees a day and can occupy either of two signs on this date, so a morning birth and a night birth may not match. The Ascendant runs through all twelve signs in twenty four hours and is sensitive to the latitude of the birthplace as well as the clock.
The widget above takes those inputs and returns the exact solar degree rather than a range, reports the distance to the nearest boundary, and computes the placements a date cannot supply. On this date the boundary distance is comfortable; on the ones just after it, that number becomes the whole answer.
Questions people ask about this
Is 21 August Leo or Virgo?
Leo in every year. The Sun reaches around twenty eight or twenty nine degrees of Leo at most, and Virgo does not begin until thirty.
Am I on the Leo Virgo cusp?
No, and no birthday is. A cusp describes missing information rather than a mixed state, and on this date the information is not missing: the position is over a degree short of the boundary.
Could a late evening birth reach Virgo?
No. The Sun covers under a degree in a full day, so even the highest years with a birth close to midnight stay inside Leo.
What decan is 21 August in?
The third decan of Leo, from twenty degrees to the sign's end, ruled by Mars in both of the schemes still in use.
Does a late Leo degree mean I have Virgo traits?
There is no mechanism by which it would. What a late degree does affect is which points in other signs the Sun aspects, and possibly which house it falls in, neither of which is a matter of borrowed sign qualities.
Which dates after this one are genuinely uncertain?
The days around the twenty second and twenty third, where the ingress itself falls. There the year, the hour and the time zone all have to be taken into account.