To understand why the Mayan culture regarded the cycle of Venus rising and setting near the Sun as key for the world, you have to understand astrology and its dating methods. Many people hear things like the Mayan calendar ends at December 2012 and falsely believe that "the world is coming to an end" as if somehow this is true.
Then, rumors spread among those who lack the knowledge of astrology in the same manner as those who hear the name Nostradamus, but have never actually read his book of astrological prophecies, but rather "interpretations" of them, often from commentators who are not versed in the applied science of astrology.
In order to know something about serious astrology and not the "pop-culture astrology" many falsely think is the same thing - it is important to understand that astrology in its true practice is one of the most serious sciences a person can practice.
The Mayan astrologers recorded Venus transits with astonishing accuracy. This, combined with the “coincidence” that the second Venus retrograde and transit in this century will occur eight years from now in May & June 2012 — just as the Mayan Calendar comes to an end - gives the uneducated in astrology a reason to spread rumor about "the end of the world." This is common among those who do not study, nor understand how to read the celestial skies, and to study the cycles of time - which the science of astrology has done for millenia.
Mayan astrologers wrote that a renewed world of new consciousness will be born on the occasion of the Venus Passage across the Sun of 6 June 2012. The intervening years between June 2004 and 2012 represent a transition period when the forces leading up to the major transformation will rapidly develop and be assimilated into human consciousness among those who use their free will to develop postively, and not negatively in the world.
Astrological Facts
Venus Transits occur when the planet Venus, the Earth and the Sun are all in a direct line. Venus is also at the same declination as the Sun. This occurs when Venus is very close to its own Nodes; that is, the points where the orbit of Venus intersects with the plane of the ecliptic (the plane in which the Earth orbits the Sun).
These transits take place in a curious 243-year repeating pattern, beginning with two transits in early December, eight years apart. Then, after a wait of 121½ years, Venus makes two transits of the Sun in early June, again eight years apart. Then there is a wait of 105½ before the entire pattern repeats.
The Mayans, like many other ancient cultures, created an astrological long count calendar with a end date in mind, because December 21st, 2012 also represents an extremely close conjunction of the Winter Solstice Sun entering tropical Capricorn with the crossing point of the Galactic equator and the ecliptic. This is what the ancient Mayans recognized as the "Sacred Tree."
Many events can take place between such Venus Transits - world events, extreme weather events, and personal, according to individual transits. In Classical Astrology, their are a host of influences related to planetary motions, including the cycles of all the planets like Venus.
The ancient Mayans also called the Venus Transits over the Sun as the "Return of Quetzalcoatl" - a messenger of light, love and wisdom - a symbol of death and rebirth of a higher consciousness. According to astrological principles we can expect the next eight years until the transit of 2012 to be a time of crisis and turmoil where the old model of separative ego-driven social structure and consciousness undergoes radical changes of a manner of being whose usefulness has passed with new spiritual and harmonious ways of being, thinking and relating is born among those with insight.
The decade of the 2010s is highlighted. Often, Venus Transits are signs of combining planetary transits ahead of time and astrologers know that the decade of the 2010s hold many new cycles that can be difficult for people who have not prepared ahead of time. The Mayan calendar notes the astrological motions of the decade only because their astrologers went as far as their astrologers could go from their time - and that was into the year 2012 due to their astroligical tracking of the transits of Venus.
For more see http://www.levity.com/eschaton/Why2012.html