Where the decan sits
A decan is a ten degree slice of a sign, so the second one begins at ten degrees of Taurus and ends at twenty. It is the part of the sign the Sun reaches after the first week and a half.
In calendar terms the Sun crosses it from around 30 April to around 10 May. Treat those dates as approximate. The tropical year is not a whole number of days, so every boundary drifts, and in practice a decan edge can fall up to a day and a half either side of the date you see printed. Someone born on 30 April or 10 May cannot settle the question from a table.
Written out in full the boundary is a position rather than a day: the decan begins the instant the Sun's tropical longitude passes forty degrees and ends at fifty. Nothing visible happens at that point, and no ancient source claims anything does. The division is a convention for describing a sign in smaller pieces, which is what makes the exact degree the only sensible way to answer the question for a birth date sitting near an edge.
Why the schemes disagree here
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, and repeats, starting from zero degrees of Aries. Counting forward through the decans of Aries and the first of Taurus lands the Moon on this one.
The triplicity scheme ignores that sequence. It hands each decan of a sign to a sign of the same element, taken in order, so the earth decans go Taurus, Virgo, Capricorn. The middle decan of Taurus therefore belongs to Virgo, and through Virgo to Mercury.
Moon and Mercury are not close relatives, and the readings that follow from them pull in different directions. Most popular articles quote one answer without naming the scheme behind it, which is where the confusion usually starts. Both are given here so you can see which one you are being handed elsewhere.
What the tradition reads into it
The Virgo colouring is read as Taurus with an eye for detail. The sign on its own is described as steady and slow to move; the Virgo overlay is taken to add sorting, measuring and a preference for things that work rather than things that merely look settled. Where the first decan of Taurus is read as accumulation, this one is read as maintenance.
The Chaldean planet is taken to add something else again. The Moon is associated with habit, appetite and the domestic, and its presence here is read as attachment to the familiar and a strong physical memory: places, food, textures, routines that are hard to interrupt. Read together, the two point at a person who keeps a system running by feel as much as by rule.
None of that is measurement. Decans are a genuine and old part of the tradition, descended from Egyptian star lists that divided the night by rising stars, but they are a second-order refinement laid over the signs rather than a foundation under them.
Beyond the Sun, and checking your degree
Every placement in a chart sits in some decan, not just the Sun. Astrologers apply the technique to the Ascendant most of all, since the rising degree is the most precise point in a chart, and to the Moon nearly as often.
The check above computes your actual solar degree from your birth date rather than matching you to a range. If you were born near either end of this decan that is the only way to answer the question, because the range itself moves between years.
Questions people ask about this
What dates cover the second decan of Taurus?
Approximately 30 April to 10 May. The edges move by up to a day and a half depending on the year, so a birth date at either end needs the actual solar degree rather than a table.
Who rules the second decan of Taurus?
It depends on the scheme. The Chaldean order gives it to the Moon. The triplicity scheme gives it to Virgo and so to Mercury. Neither is more official than the other, they are simply different systems.
Why do other sites say Mercury and not the Moon?
Because they are using the triplicity scheme, usually without saying so. Virgo is ruled by Mercury, so a Virgo decan gets reported as a Mercury decan. The Chaldean answer here is the Moon.
What does the Virgo influence add to Taurus?
The tradition reads it as precision applied to something already patient. Taurus supplies the steadiness and Virgo supplies the sorting and correcting. It is an interpretation, not a finding.
Do decans apply to the Ascendant?
Yes, and the Ascendant is arguably the best use of them, because the rising degree is calculated to the minute from your birth time. The Moon is also commonly read by decan.
Is a decan reading more accurate than a sign reading?
It is more specific, which is not the same thing. Narrowing from thirty degrees to ten sharpens the description, but it does not make the underlying claim any more testable.