Three bands, not one sign
Leo covers thirty degrees and divides at ten and at twenty. The Sun takes about ten days to cross each band, so the sign's month splits into three roughly equal stretches with different rulers attached. A birth in late July, one in early August and one in mid August fall in different bands, and older practice treats that as a real distinction rather than a refinement.
At noon in every year from 1930 to 2035 the Sun on 28 July has stood between roughly four and a half and six degrees of Leo. That is the middle of the opening band, several days clear of the change at ten degrees, so the face is as settled as the sign. A birth at any hour on this date returns the same band, which is not the case for dates sitting near ten, twenty or thirty degrees.
The rulers of the opening band
Two schemes assign that band and they do not agree. The Chaldean order runs the visible planets by apparent speed and cycles them through all thirty six faces from the start of Aries, arriving here at Saturn. The triplicity scheme gives the three faces of a sign to the three signs of its element in turn, which leaves the first face of Leo with Leo, and therefore with the Sun.
Compare that with the later bands. Under the Chaldean count the second face of Leo goes to Jupiter and the third to Mars, while the triplicity scheme gives them to Sagittarius and Aries. Whichever scheme you follow, a 28 July Leo and a mid August Leo answer to different planets at this layer, which is precisely the specificity a sign label cannot offer. Three bands across twelve signs give thirty six categories rather than twelve, and the finer grid costs nothing once the degree is in hand.
What the tradition attaches to it
Writers following the Chaldean order describe the opening degrees of Leo as warmth meeting some form of restraint, since Saturn and the Sun sit at opposite ends of that scheme. Those following the triplicity version describe the same degrees as Leo undiluted, sign and face pointing at one planet.
Both are conventions carried forward in writing, not measurements, and this page reports them as such. Neither says anything about how a life will go, and the site's position is that a face is a coordinate band with a literature attached, which is interesting and is not a forecast.
The layers a date cannot supply
Faces apply to every placement, not only to the Sun. Your Moon sits in a face of its own and your ascendant in another, and both need the hour and the place before they can be found at all. The Moon moves about thirteen degrees a day, so it crosses more than a face inside a single birthday, and the ascendant moves through a whole sign every couple of hours.
The widget above takes your year, your hour and your birthplace and returns your exact solar degree instead of the noon figure quoted here, together with your distance from the nearest sign boundary. On this date that reading comes back at several days on the Cancer side, which settles the sign and leaves the interesting work still to do.
Questions people ask about this
What decan is 28 July?
The first decan of Leo, from zero to ten degrees, with the Sun sitting around four and a half to six degrees depending on your birth year.
How does that differ from a mid August Leo?
A mid August birth falls in the second or third decan, which carries a different ruler under both schemes. The sign is shared and the band is not.
Who rules the first decan of Leo?
Saturn in the Chaldean face order, the Sun in the triplicity scheme. Both are in current use and neither has been shown to supersede the other.
Is 28 July ever anything but Leo?
No. Every year sampled at noon from 1930 to 2035 gives Leo, with the Cancer boundary about five days behind and beyond the reach of any birth hour.
Does my Moon have a decan too?
Yes. Every placement sits at a degree and therefore in a face. Finding the Moon's requires your birth hour and birthplace as well as the date.
Why is the decan missing from most birthday pages?
Because it needs a computed degree rather than a date range, and most pages never leave the range. Once the degree is known the face follows at once.