Precession of the Equinoxes
The winter solstice of 2012 or December 20, 2012 is the end of the Mayan Calendar. According to my understanding it is also the beginning of the Age of Aquarius. At that time the winter solstice will align with the plane of the Milky Way galaxy to which our solar system belongs. I believe this date is important for other reasons as well, having to do with the Pleiades and the Star Regales changing signs in the tropical zodiac due to the precession of the equinoxes.
To understand how this works it is first necessary to understand what precession of the equinoxes is and what causes it. First of all there are two zodiacs used in astrology. They are the tropical zodiac and the sidereal zodiac. Both zodiacs are arranged along the ecliptic which is the plane in space defined by the Earths orbit around the Sun. The tropical zodiac which is primarily used in western astrology divides the ecliptic into 12 signs of 30 degrees each referenced to the equinoxes and solstices. The sidereal zodiac which is primarily used in eastern astrology is made up of actual constellations of stars that are arranged along the ecliptic. A little over 2000 years ago these two zodiacs perfectly lined up together and the signs of the same name occupied the same positions on the ecliptic. Since then they have been drifting apart due to precession of the equinoxes.