The one thing the date decides
Give only a day and a month and exactly one placement follows. On 31 July the Sun stands between about seven and a quarter and just under nine degrees of Leo, taken at noon across every year from 1930 to 2035, and no birth hour moves it out of that band.
The reason the figure holds so steady is the leap year rule. Each ordinary year the date arrives a quarter of a day early against the Sun's return, and every fourth year the extra day in February hands that time back, so the position oscillates around a fixed point instead of wandering.
That single fact is worth stating precisely rather than vaguely, because the precision is available and costs nothing. What follows is the list of everything the date does not carry, which is considerably longer.
The Moon is already unknown
The Moon moves about thirteen degrees a day, which is roughly half a sign. On a given 31 July it may begin the day in one sign and end it in the next, so two people born on the same date twelve hours apart can have different lunar signs.
Across the century the Moon has occupied every sign in turn on this date, since its cycle bears no relation to the calendar year. That is why a birthday alone can never state it, and why any page that assigns a moon sign to a date is inventing one.
Nothing about that is a defect in the system. The Moon simply moves too quickly for a calendar date to constrain it, in the same way a date cannot tell you the hour of a tide.
The rising sign is entirely open
The Ascendant is the point of the ecliptic climbing over the eastern horizon at the moment of birth, and the whole zodiac passes that point in twenty four hours. Any of the twelve can rise on 31 July depending on the minute.
Latitude complicates it further. The signs do not rise at even intervals, and the further from the equator the birthplace, the more unequal they become, so some signs take well over two hours to rise and others under one. The houses follow the Ascendant, which means the whole framework of the chart hangs on information the date does not carry.
This is also why two siblings born on the same date in different years can have charts with almost nothing in common apart from the Sun.
An approximate time is still better than none. Even a window of a few hours removes most of the twelve possibilities, and the widget above will work from an estimate as readily as from a certainty.
Where that leaves the sun sign
What the date does add is the decan. Eight degrees falls in the first ten degree band of Leo, which the Chaldean order assigns to Saturn, and the tradition reads that as the sign's warmth working under a stricter frame. That is convention rather than measurement, but the band itself is fixed by the degree.
Taken together, the date supplies a sign, a degree and a band, and withholds the Moon, the Ascendant and every house. The widget above closes the gap: it computes the solar degree for your own year and birthplace, and reports how many degrees separated your birth from the nearest sign boundary.
Stated that way the date is useful and small at the same time, which is the honest description of what a birthday can do on its own.
Questions people ask about this
Is 31 July Leo or Cancer?
Leo, without exception. The Sun crossed into Leo eight or nine days earlier, and the ingress never falls anywhere near the end of July, so no year produces a Cancer birth on this date.
Can my moon sign be worked out from the date?
No. The Moon changes sign every two and a bit days and can change during the day itself, so an hour of birth is needed. Any page that names a moon sign from a date alone is guessing.
What rising sign goes with 31 July?
Any of the twelve. The Ascendant depends on the clock time and the latitude of the birthplace, not on the calendar date, and it turns through the entire zodiac in a day.
How exact is the solar degree here?
The noon figure sits between about seven and a quarter and just under nine degrees of Leo depending on the year. Adding your birth hour and place gives a single figure rather than a band.
Does the decan need a birth time too?
Not on this date. The whole day sits inside the first ten degree band of Leo, so the decan is settled by the date alone even though the exact degree is not.
If the date settles so little, what is it good for?
It fixes the Sun, the degree to within a small margin and the decan. That is three specific things, which is more than a sign name, and it is honest about the rest.