Precession
In the solar system, the planets and the Sun share roughly the same plane of orbit, known as the plane of the ecliptic. From our perspective on Earth, the Zodiacal constellations move along or near the ecliptic, and over time, appear to recede counterclockwise by one degree every 72 years. This movement is attributed to a slight wobble in the Earth's axis as it spins. As a result, approximately every 2160 years, the constellation visible on the early morning of the spring equinox changes. In Western astrological traditions, this signals the end of one astrological age (currently the Age of Pisces) and the beginning of another (Age of Aquarius). Over the course of 26,000 years, the precession of the equinoxes makes one full circuit around the ecliptic.
Just as the spring equinox in the northern hemisphere is currently in the constellation of Pisces, so the winter solstice is currently in the constellation of Sagittarius, which happens to be the constellation intersected by the galactic equator. Every year for the last 1000 years or so, on the winter solstice, the Earth, Sun and the galactic equator come into alignment, and every year, precession pushes the Sun's position a little way further through the Milky Way's band.