What is settled
Every year from 1930 to 2035, taken at noon, puts the Sun on 13 May between about twenty one point eight and twenty three point two degrees of Taurus. The sign is not in question, because the boundaries at either end of Taurus are days away. The decan is not in question either: the whole range lies above twenty degrees, inside the final band of the sign.
The narrowness of that range is itself the point. The calendar and the orbit stay in step through the leap year cycle, so a given date returns to nearly the same solar position year after year. Your grandparents born on this date had a Sun within a degree or so of yours.
What the hour decides
The Moon covers roughly thirteen degrees of the zodiac in a day, a little more than one third of a sign. Over the twenty four hours of any 13 May it therefore travels far enough to change sign, and which sign you get depends on the hour of birth. Two Moon signs are usually in play and the date alone cannot pick between them.
The rising sign is worse, or better, depending on how you look at it. The Earth turns once a day, so the entire zodiac passes the eastern horizon in that time and the rising sign changes roughly every two hours. All twelve are possible on 13 May. Without a birth time there is no rising sign to report, and everything built on it, meaning the whole house structure, goes with it.
Time zone matters here as well. The Moon's sign changes at one global instant, and only the local clock differs, so two births recorded at the same wall clock time in London and in Los Angeles can fall on opposite sides of it. This is the most common reason a Moon sign is reported wrongly, and it disappears once the birthplace is included.
The layers the date does fix
The Sun in Taurus is fixed earth under Venus, and the tradition reads that as steadiness, physical comfort and a long fuse. The final decan of Taurus belongs to Saturn in the Chaldean order and, by way of Capricorn, in the triplicity scheme too. The tradition adds restraint and an interest in what lasts.
That is the whole of what a date supports. It is more than most birthday pages give you, because they rarely mention the degree or the decan at all, and it is still a long way short of a chart.
Why the sun sign is the least personal part
Roughly a twelfth of everyone alive shares your Sun sign, and every person born within a day or two of you shares your solar degree as well. Nothing in that is specific to you. What makes a chart individual is the combination of placements, and most of that combination depends on the hour.
This is why the standard birthday page reads the same for everyone. It is working from the one factor that varies least. If you want the part that actually distinguishes your chart, the birth time is the piece to go and find, and a birth certificate or a hospital record is usually where it is.
None of this makes a birthday page useless, but it does set the limits of one. What a date can honestly deliver is a sign, a degree, a decan and a clear account of what remains unknown. Anything past that is either borrowed from the sign description or invented, and it would read the same for the several million people who share the date.
Questions people ask about this
Is 13 May Taurus or Gemini?
Taurus. The Sun crosses into Gemini around 21 May, so this date is roughly eight days clear of the boundary in every year, at every hour and in every time zone.
What exactly does the date settle?
The Sun sign and the solar degree, which for 13 May sits near twenty two of Taurus, and therefore the decan. Nothing else in a chart follows from a date on its own.
How many Moon signs are possible on this date?
Usually two, because the Moon travels about thirteen degrees a day and can cross a sign boundary partway through. Which one applies to you depends on the hour and, in edge cases, on the birthplace.
Can the rising sign be guessed from the date?
No. The horizon works through all twelve signs every twenty four hours, so the date carries no information about it. Only the time and place of birth can narrow it.
What if I do not know my birth time at all?
The Sun, and so the sign and decan, can still be given accurately. The Moon can often be narrowed to two candidates. The rising sign and the houses cannot be reported honestly without a time.
Does the solar degree matter in practice?
It decides the decan and it decides which aspects the Sun makes to other placements, since aspects are measured in degrees. For the sign label alone it makes no difference.