Past the boundary in every year
At noon across the last century the Sun on 20 February has been between about half a degree and just over two degrees of Pisces. The ingress happens between one and two days earlier depending on the year, so even the earliest hour of this date has always been on the Pisces side.
That is the practical difference between this date and the two before it. On 18 and 19 February the change can fall inside the day itself, which makes the hour decisive. By the twentieth the question has closed.
The precise instant of the change shifts by roughly six hours each year and jumps back nearly a day whenever February gains a twenty ninth, which is why the ingress wanders across a two day span rather than sitting on a fixed date.
What the first decan carries
The opening ten degrees of Pisces form the first decan, and in the Chaldean order of planetary rulers that band is assigned to Saturn. It is an unexpected pairing at first glance, since Pisces is mutable water ruled by Jupiter in the classical scheme and by Neptune in the modern one, while Saturn is the planet of limit and structure.
The tradition reads the combination as the more disciplined and self contained part of the sign, where receptivity comes with a boundary around it. That is inherited interpretation rather than measurement, and other decan schemes assign this same band to Jupiter by following the triplicity instead, so the disagreement is worth knowing about.
Pisces itself is mutable water, and the sign is usually described as receptive, adaptable and inclined to blur its own edges. The first decan is read as the exception within that, which is part of why the decan layer is worth knowing at all.
Why an early degree is not a weak one
A Sun at one degree of Pisces is entirely in Pisces. There is no gradient at the start of a sign and no residue of the previous one, whatever the popular writing about cusps suggests. The word describes a gap in someone's information rather than a mixed state in the sky.
The early degree does have real consequences elsewhere. It changes which planets the Sun aspects, and it means a solar return falls at the very beginning of the sign, but neither of those affects the sign attribution itself.
What the date leaves for a calculation
The Moon moves about thirteen degrees a day and changes sign every two and a half days, so on any 20 February it sits in one of two signs depending on the hour and the time zone. The rising sign is fully open, turning through the whole zodiac each day and varying with the latitude of the birthplace.
The widget above computes the solar degree from your own year, time and place rather than the noon figure used here, and reports how far your birth fell from the nearest sign boundary. Here that will come back at a day or two, which is enough to be firm.
If the birth hour is unknown the calculation falls back to noon. On this date that costs nothing at the level of the sign, though it leaves the degree loose by up to half a degree and the horizon entirely unresolved.
Questions people ask about this
Is 20 February Aquarius or Pisces?
Pisces. The Sun enters the sign around 18 or 19 February, so by the twentieth the change has already happened in every year of the modern calendar.
What degree of Pisces is this?
Between about half a degree and two degrees, depending on the year and the hour. The widget above gives the exact figure from your birth details.
Which decan and which ruler?
The first decan of Pisces, from zero to ten degrees, which the Chaldean sequence assigns to Saturn.
Why does Saturn rule part of Pisces?
Because the Chaldean scheme runs the planets in order of apparent speed continuously around the zodiac from the first degree of Aries. Saturn simply falls in that slot, independently of the sign's own ruler.
Am I on a cusp?
No. The Sun holds a single longitude at any moment and here it is inside Pisces. Cusp is a word for missing information rather than a blended sign.
Does an early degree change anything?
It changes the aspects the Sun makes and the point at which a solar return falls. It does not make the sign apply any less.