The divisions everyone agrees on
A sign covers thirty degrees and splits into three sections of ten. For Libra the cuts fall at ten and twenty degrees, giving an opening band, a middle band and a closing band. This date sits near the top of the middle one in every year of the range, well clear of the twenty degree line.
Nothing about that is in dispute. The bands are arithmetic, and any tradition that uses decans at all uses the same boundaries. The disagreement starts at the next step, when a planet is attached to each band. Two traditions can place a birthday in the same ten degrees and then describe it in quite different terms, which is worth checking before taking any decan description at face value.
Two ways to assign the ruler
The Chaldean scheme runs the seven traditional planets in the order Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon, and lays that sequence continuously across all thirty six decans of the zodiac starting from the beginning of Aries. It ignores sign rulership entirely, which is how the middle band of a Venus ruled sign ends up assigned to Saturn.
The other common approach is triplicity based. It gives each decan of a sign the ruler of one of the three signs sharing its element, taken in order. For the middle band of Libra that means Aquarius, so the ruler comes out as Saturn in traditional terms or Uranus in modern practice. On this particular band the two schemes happen to agree if the older rulerships are used, which is not true for most decans.
What is read into the band
Venus rules the sign as a whole and is described as the principle of relation and proportion. Saturn over the middle third is described as limit and standard. Traditional readings put the two together as a stretch of Libra where the weighing has a threshold in it, so a judgement is reached and then held rather than adjusted to keep the peace.
These are attributions from a scheme, not findings. They are worth knowing because they are what the tradition actually says about this position, and because a page that stops at the sign name never gets far enough to say anything specific at all.
The rest of the chart is still open
The Moon moves about thirteen degrees a day, so this date allows two sign placements for it and sometimes three. The rising sign is not constrained by the date in any way, since a full rotation carries all twelve signs across the eastern horizon and only the hour and the latitude of the birthplace pick out one of them.
The widget above works from those details rather than the calendar square, giving the exact solar degree for the year and birthplace and the distance from the nearest sign boundary. On 11 October that distance is comfortable, so the sign was never in question and the interesting numbers are the ones further down. Knowing which scheme a description came from is part of reading it properly, because two sources can describe the same ten degrees of longitude in quite different terms and both be reporting their tradition accurately. The band itself is fixed here, so the only open question is which planet a given source hangs on it.
Questions people ask about this
Which decan is 11 October?
The second decan of Libra, from ten to twenty degrees. The Sun sits near eighteen degrees on this date in every year from 1930 to 2035, so the band holds regardless of the year or the hour.
Why do sources give different decan rulers?
Because two schemes are in use. The Chaldean order runs the traditional planets in a fixed sequence across all the decans, while the triplicity method assigns each band the ruler of a sign sharing its element. They agree only sometimes.
Which scheme should I use?
Traditional practice generally uses the Chaldean order, and it is the one applied on this site. The triplicity method is common in modern writing, so it is worth knowing which one a description you read is based on.
Is 11 October close to the third decan?
Not close enough to matter. The twenty degree line arrives around the thirteenth of the month, so this date has roughly a degree and a half of margin in the tightest year.
Does the decan change the sun sign?
No. The decan is a subdivision inside the sign, so the Sun is fully in Libra whichever band it occupies. The decan ruler adds a second layer of description rather than replacing the first.
What does the date not tell me?
The Moon sign, the rising sign and every house placement. Those depend on the hour and the birthplace, and between them they account for most of what distinguishes one chart from another.