A settled position
At noon in every year from 1930 to 2035, the Sun on 12 May has stood between about twenty point eight and twenty two point two degrees of Taurus. There is no ambiguity here of any kind: not about the sign, since both boundaries are more than a week away, and not about the decan, since the twenty degree line sits below the whole range.
That stability is worth stating because it is unusual to see it stated. The published date ranges that assign signs to birthdays are rounded, and rounding is where most of the confusion about birthdays comes from. A calculated degree removes the rounding.
It is also why the hour of birth makes so little difference to the sun sign on a date like this. The Sun moves less than a degree in a full day, so the gap between a birth at midnight and one twenty four hours later is smaller than the year to year variation, and both sit in the same decan of the same sign.
The Chaldean order and where Saturn comes in
The decan scheme most often called Chaldean walks the seven visible planets in order of apparent speed, slowest first, and deals them out around the zodiac in ten degree steps beginning at Aries. Saturn, Jupiter, Mars, the Sun, Venus, Mercury, the Moon, then round again. Starting from the first decan of Aries and counting forward, the band from twenty to thirty degrees of Taurus lands on Saturn.
The triplicity scheme reaches the same answer by a different route. It assigns the three decans of a sign to the three signs of its element in order, so Taurus takes Taurus, Virgo and Capricorn, and Capricorn is ruled by Saturn. When two schemes built on different logic agree, the reading rests on firmer ground than usual.
What the tradition reads into it
Taurus itself is fixed earth under Venus, and the standard reading is steadiness, an eye for value, and reluctance to be hurried. Saturn over the last third is read as adding structure, patience with long timescales and a certain economy. The older material describes this band as the part of Taurus most interested in permanence.
Mark all of that as interpretation. The degree is a measurement and can be checked against an ephemeris; the character sketch attached to it is a tradition, and different traditions attach different sketches. The site's position is to keep the two labelled separately.
The sign ruler sits above the decan ruler in that structure. Venus governs Taurus and Saturn governs the band, so the tradition reads a layered condition rather than a single quality, and the weight given to each depends on where Venus and Saturn themselves fall in the chart. That is a calculation, and it needs more than a date.
One placement, not a personality
A birth date fixes the Sun and very little else. The Moon moves about half a sign in a day, so a 12 May birth has two candidate Moon signs until the hour is known. The rising sign has twelve candidates, since the horizon works through the entire zodiac in twenty four hours and changes roughly every two.
House placements follow the rising sign, so they are unknown too. That means the areas of life a chart assigns to each planet are unknown. Everything a birthday page confidently tells you about your character is built on the one placement that everybody born that month shares, which is a strange foundation for a description of an individual.
Questions people ask about this
What sign is 12 May?
Taurus, in every year from 1930 to 2035 and at every hour of the day. The nearest sign boundary is the crossing into Gemini around 21 May, which is more than a week away.
What degree of Taurus is a 12 May Sun?
Around twenty one, with the full range across a century running from about twenty point eight to twenty two point two. Your own figure depends on your year, hour and birthplace.
Which decan is that?
The third decan of Taurus, covering twenty to thirty degrees. The whole 12 May range sits above the twenty degree line, so the decan is not in doubt for this date.
Do the decan schemes disagree here?
Not for this band. The Chaldean order gives it to Saturn and the triplicity scheme reaches Capricorn, which is also ruled by Saturn. Elsewhere in the zodiac the two schemes often differ.
Is a Saturn decan harder than the others?
The tradition reads it as sober and long sighted rather than as unlucky. Nothing in a decan is treated here as good or bad fortune, and the reading would shift with the rest of the chart.
What would change if I found my birth time?
The Moon sign would be settled, the rising sign would be settled, and every house placement with it. The Sun would move by a fraction of a degree and stay exactly where it is otherwise.