Pisces is the Fishes, two fish tied by their tails at the star Alrescha, the Knot. The March equinox point, where the Sun crosses the celestial equator heading north, has lain inside Pisces for roughly two thousand years and will stay there for some centuries yet. That is why the Sun is in Pisces from March into April, while the horoscope’s Pisces has finished by the equinox. Around 27 March the Sun’s disc also brushes the northern edge of Cetus, the Sea-Monster, without its centre ever leaving Pisces.
The dates above are computed for 2026 from the constellation boundaries the International Astronomical Union fixed in 1930. They move by about a day every seventy years, so they are good for a lifetime, and for the particular day you were born the calculator will check the boundary itself.