Three hundred and thirty degrees, on 18 or 19 February
The last sign boundary before the zodiac closes falls at three hundred and thirty degrees of ecliptic longitude, which the Sun reaches on 18 or 19 February. Saturn's fixed air gives way to Jupiter's mutable water, and the wheel's final month begins.
Of the twelve ingresses this is the earliest in its month, and it sits in the one month whose length varies. The crossing still moves for the usual reason: about six hours of slippage a year, cancelled by the leap day, so the instant sweeps over roughly a day and a half across the cycle.
Both rulers hold two signs
Saturn takes Capricorn and Aquarius in the traditional scheme; Jupiter takes Sagittarius and Pisces. So on either side of this line is a sign whose ruler is already committed elsewhere, and both of those partners sit in the winter quarter as well. It is a tidier piece of structure than the boundary myth built on top of it.
The myth is simply wrong on the geometry. One solar longitude belongs to one thirty degree arc, so a birth is in Aquarius or in Pisces, never in a blend of the two.
The word cusp, used correctly
Astrology reserves it for the boundaries of the houses, which are cut from the horizon and meridian of the place a birth occurred and rotate through the whole zodiac every day. Sign boundaries are fixed arcs measured along the ecliptic and are indifferent to the birthplace entirely. The two divisions answer different questions and only one of them has cusps.
Either side of the line
Aquarius closes under the Moon in the Chaldean decans, the most receptive band of the least receptive sign, and that reading has a stated cause that owes nothing to Pisces.
Pisces opens under Saturn, which is why the first degrees of the sign are described as more contained than the rest of it. Venus is exalted at twenty seven degrees of Pisces, right at the far end of the season, so the most celebrated point in Pisces is as far from this boundary as it could be.
Deciding a birth in mid February
An ephemeris takes the year, the clock time and the place, and returns the Sun's longitude to a fraction of a degree. Compare it with three hundred and thirty and the sign follows immediately.
Because the ingress is one worldwide instant while the date on a birth certificate is local, the same moment is the 18th in some time zones and the 19th in others. That alone explains most of the conflicting ranges in circulation.
Questions people ask about this
Am I a Aquarius Pisces cusp?
No, because that placement does not exist. The Sun was either short of three hundred and thirty degrees at your birth or past it, which makes you Aquarius or Pisces.
What date does the Sun move from Aquarius to Pisces?
18 or 19 February, depending on the year and on the time zone the birth is dated in. The instant is the same worldwide; only the local calendar date differs.
Do I have traits of both signs?
Popular astrology says so and sells the blend as a type. The chart does not show it: one Sun, one longitude, one sign. Other placements can of course sit in either sign.
What is a cusp in real astrology?
A house boundary. Houses are divided using the horizon and the meridian of the birthplace, and their cusps are genuine dividing lines within a chart.
How close to the boundary do I need to be to bother checking?
About forty eight hours either side. Inside that window the year and the birth time decide the answer; outside it, any date range gives the same result.