Why the generic pages all match
A sun sign table is a list of date ranges, copied from source to source, that assigns 21 September to Virgo. The assignment is correct on this date. What the table cannot do is say where in Virgo, because a table has no room for a number that changes with the year and the hour.
So the pages fall back on adjectives. Tidy, practical, critical, helpful. The adjectives are the same on every site because they come from the same handful of nineteenth and twentieth century sources, and none of them are specific to this date rather than to any of the thirty odd days that share the sign.
The position, which is specific
Taken at noon across 1930 to 2035, the Sun on 21 September lies between 27.6 and 29.0 degrees of Virgo. It is inside the final degree band of the sign in the later years of that span and just short of it in the earlier ones, and it has not crossed into Libra at noon in any year of the range.
The narrowness of that spread is itself the answer to a common question. The Sun does not arrive on a date at a random place each year. The four year leap cycle keeps the calendar and the orbit roughly in step, so the same date returns the Sun to nearly the same longitude for a century at a time, drifting a few tenths of a degree later in each ordinary year and recovering when February gains a day.
The residual movement in that figure has a shape worth knowing. Within each four year cycle the position creeps later in the three ordinary years and drops back when February gains its day, so a birth in a leap year finds the Sun slightly behind where a birth three years later would find it. Over a full century the pattern also tilts a little, because the leap rule slightly overcorrects and century years mostly skip the extra day.
The decan and its ruler
The band from 20 to 30 degrees of Virgo is the third decan, and this date is inside it in every year. In the Chaldean sequence that decan belongs to Mercury, which is also the ruler of the sign, so the two layers point at the same planet rather than at different ones.
Under the alternative triplicity method the same band is called the Taurus decan of Virgo and takes Venus as its sub ruler. The degrees are identical either way. Only the tradition being applied to them differs, and any page that names one scheme without mentioning the other is presenting a convention as though it were a fact.
What being near the end does and does not mean
The equinox that opens Libra falls a day or two after this date, and its exact moment moves around within that window from year to year. That proximity is worth quantifying rather than dramatising. The widget above reports the distance from the boundary in hours, and on this date the answer runs to a day or more even in the closest years.
A Sun in the twenty ninth degree of a sign is fully in that sign. Some traditions attach significance to the final degree of any sign, calling it a place of completion or of urgency, which is an interpretation and is marked as one here. The technical position is unchanged: the sign applies in full until the longitude crosses the boundary.
A practical consequence for anyone born on this date is that a table and a calculation agree here, and no birth time is needed to establish the sign. The birth time earns its keep elsewhere, on the Moon, which has two candidates on any given year, and on the Ascendant, which has all twelve until the hour and place are known.
Questions people ask about this
Is 21 September Virgo or Libra?
Virgo. In every year from 1930 to 2035 the Sun at noon on this date is still in Virgo, since the equinox arrives on the twenty second or twenty third.
How close to Libra is 21 September?
Between one and two and a half degrees, which translates to somewhat more than a day of solar motion. The widget above converts that into hours for your own year.
What is the last degree of a sign called?
It is often referred to as the anaretic or critical degree, the twenty ninth. Various meanings have been attached to it over the centuries, all of them interpretive rather than measurable.
Which decan is this date in?
The third decan of Virgo, covering 20 to 30 degrees. The Chaldean order gives it to Mercury and the triplicity order calls it the Taurus decan under Venus.
Why do all the birthday pages say the same thing?
Because they are all reading from the same date table and the same inherited list of sign adjectives. A table cannot record a position that varies, so the variable part gets dropped.
Does the birth hour change my sign on this date?
No. The Sun covers about a degree a day, so even the full span from midnight to midnight leaves it inside Virgo in every year of the range.