One step past the line
At noon from 1930 to 2035, a 1 May Sun sits between ten point two and eleven point six degrees of Taurus. The first decan ends at ten. Even the earliest position in that range clears it, which makes this the first date in the calendar where the second decan of Taurus is a safe answer rather than a likely one.
The day before is genuinely split across the boundary. This one is not, though the margin at the low end is only a fraction of a degree, and that is worth saying rather than glossing over.
Put another way, the Sun crosses into the middle third of Taurus somewhere in the last hours of 30 April in the earliest years and somewhere in the middle of it in the latest, and by the first of May the crossing is behind it either way.
The middle decan and who rules it
The second decan of Taurus covers ten to twenty degrees. Under the Chaldean order of faces, counted without a break from the start of Aries through Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter, this band falls to the Moon. Under the triplicity scheme, which hands each decan of a sign to a sign of the same element in order, it falls to Virgo and through Virgo to Mercury.
Those are not neighbouring ideas. The Moon is read as receptive and habitual, Virgo as discriminating and precise. A page that names a single decan ruler for 1 May without saying which count it used has taken a side without telling you.
Both readings still sit on top of a Venus ruled fixed earth sign, so they modify rather than replace the description of Taurus itself. That is the usual relationship between a sign and its decan in the tradition.
What the calendar keeps steady
The solar degree for a fixed calendar date drifts remarkably little. A common year is about a quarter of a day short of the Sun's return, so the position slips roughly six hours further along the orbit each year until a leap day resets it. Across a century the net wander on 1 May is around a degree and a half, and it oscillates inside that band rather than moving steadily in one direction.
That stability is what makes a page like this possible. It is also why the tables that assign date ranges to signs and decans mostly work, and why they fail at the edges: the drift is small, but on a boundary date a small drift is all it takes.
Everything the date leaves unfixed
The Moon covers a sign in about two and a half days, so a 1 May birth has the Moon in one of two signs and the date cannot choose between them. The Ascendant is worse: the entire zodiac rises over the course of a day, so any of the twelve signs can be on the horizon depending on the hour and the latitude.
Set against that, the sun sign is the coarsest piece of information in a chart. Everyone born within a month of you shares it. The widget above computes your exact degree from your year and birthplace, and reports how far that position stood from the nearest sign boundary, which is the level at which the date starts saying something specific. It also takes the birthplace into account, since a solar position quoted for noon in one time zone is not the position for noon in another.
Questions people ask about this
Is 1 May Taurus or Aries?
Taurus, in every year sampled at noon between 1930 and 2035. The Sun crossed into Taurus around ten days earlier, so the boundary is not in play on this date.
Which decan is a 1 May birthday in?
The second decan of Taurus, from ten to twenty degrees. The lowest position recorded on this date is just over ten degrees, so the first decan is behind it in every year checked.
Who rules the second Taurus decan?
The Moon in the Chaldean order and Virgo, and so Mercury, in the triplicity scheme. Both are traditional and they disagree, which is why the scheme should always be named.
How precise is the degree figure?
The range across more than a century is about a degree and a half wide. Your own position inside that band depends on your birth year and hour, and the widget above returns it exactly.
Does 1 May tell me anything about my rising sign?
Nothing at all. The rising sign depends on the time and place of birth, not the date, and all twelve possibilities remain open until those are known.
Why do some sources put 1 May in the first decan?
Usually because they are rounding, or working from decan date ranges printed as fixed. Those ranges drift by up to a day and a half, so a date near a boundary needs a calculation rather than a table.