What is settled
The Sun on 28 December is in Capricorn, and has been in every year of the modern calendar. Its longitude at noon runs between about five point eight and seven point two degrees across the years from 1930 to 2035, so the first decan applies, the band from zero to ten degrees whose Chaldean ruler is Jupiter.
That is a genuine set of facts and more than most descriptions of the date offer. It is also the whole of what the date decides on its own. Every further placement depends on the hour and on the place, and those are not properties of a date.
It is worth being blunt about that, because the usual practice is to slide from the one settled fact into a description of a person, as though the sign carried the rest along with it. It does not, and the tradition itself has never said it did.
The Moon: two candidates, one hour
The Moon covers roughly thirteen degrees a day and changes sign every two and a bit days. On any 28 December it will therefore have been in one of two adjacent signs, and in some years it crosses between them partway through the day, so two births twelve hours apart hold different lunar signs.
Which of the pair applies is not something a general page can know, because it depends on the year and the hour together. Traditional practice reads the Moon for temperament, habit and immediate response, so this is not a minor detail being deferred. It is a placement of comparable weight to the Sun, and the date genuinely does not fix it. What the date does is cut the field from twelve to two, which is real progress and still short of an answer.
The Ascendant: all twelve remain
The rising sign is decided by which degree of the ecliptic is climbing over the eastern horizon at the moment of birth. The Earth turns once a day, so the whole zodiac passes that point in twenty four hours and any of the twelve signs can rise on any date.
The birthplace matters as much as the clock. Signs rise at unequal speeds, and the further the latitude from the equator the more unequal they become, so an identical birth time in Glasgow and in Quito produces different Ascendants. This placement is the one most people mean when they say a chart described them well, and it is the one a birthday page can say least about.
Why the sun sign is the least personal part
Roughly one person in twelve shares your sun sign, and everyone born on 28 December in any year shares your solar degree to within a fraction. That is precisely what makes the placement useful for a lookup page and weak as a description of an individual.
The combinations that are actually rare come from putting the placements together, and every one of them needs a time and a place rather than a date. The widget above starts that process: it returns the Sun's exact longitude for your own birth and how far it sat from the nearest sign boundary, which on this date is a comfortable distance in both directions.
Questions people ask about this
What sign is a 28 December birthday?
Capricorn, in every year. The Sun crossed the boundary at the solstice about a week earlier, so there is no year in which this date falls in Sagittarius.
What does the date not tell me?
The Moon sign, the rising sign and every house placement. The date narrows the Moon to two possibilities and leaves the rising sign completely open, since all twelve rise within a single day.
How close is the Sun to a boundary here?
Not close. It sits near six degrees of Capricorn, so it is several days of solar motion from the start of the sign and more than three weeks from the end of it.
Do I need my exact birth time?
For the sun sign and decan, no. For the rising sign, yes, ideally to within a few minutes, because the Ascendant advances through roughly a degree every four minutes.
What if two people share this birthday?
They share a sun sign, a solar degree and a decan. They may hold different Moon signs and will usually hold different rising signs, which is where the real difference between their charts begins.
Is the sun sign worth knowing at all then?
Yes, as a starting point. It is checkable, it is stable, and it fixes the frame that the decan and the aspects are read inside. It is simply not the whole of a chart.