The equinoxes are where the ecliptic crosses the equator.
As the Earth's axis and equatorial plane pivot under precession,
the equinox points migrate westward along the ecliptic, 360° in 25800 years or 1.4° per century.
This animation shows the vernal equinox drifting 28° in 20 centuries of precession:
Images generated by Stellarium
The Sun moves eastward (right to left) 360° per sidereal year relative to the stars along the ecliptic.
Meanwhile the vernal point moves 0.014° per year the other way, so the Sun moves that much less between vernal equinoxes - a tropical year.
This makes a tropical year 20.4 minutes (1/25800 year) shorter than a sidereal year.
The modern calendar approximates the tropical year to keep the average equinox and solstice dates from drifting.
While some traditions define a sidereal zodiac anchored to the stars, the Western zodiac is tropical, linked to the equinoxes and solstices and migrating along with them.
The sign of Aries is defined as the first 30° along the ecliptic east of the vernal equinox, even as that point moves through the constellations Pisces and Aquarius.
Similarly, the sign of Libra remains the first 30° east of the autumnal equinox despite its present alignment with the constellation Virgo.
We can blame the resulting confusion on Ptolemy, who understood precession yet named the 12 equal signs after the nearest constellations in his own lifetime anyway.
For more about the distinction between constellations and signs, see this answer.
Given a tropical year and a tropical zodiac, the Sun is always in the same sign at the same time of year.
However, as the equinoxes and solstices drift westward over the centuries, so does the Sun's position among the constellations at a given time of year.