And before it happens again, the Earth will change beyond recognition
It won't happen again until December 2117. On June 5/6th, 2012, Venus will transit the face of the Sun in an event of both historical and observational importance. The best places to watch are in the south Pacific, but travel is not required
Check on map where will be visible → http://goo.gl/iSDsT
Watch NASA video → http://goo.gl/ARlC1
See picture from the last transit → http://goo.gl/JGpWX
Layman's Terms:
The Sun, Venus and Earth all line up - a rare occurrence. It is like a solar eclipse where the moon passes in front of the sun, only this time (tiny) Venus will pass in front of the Sun (also call a transit). Venus so far away, though it is approximately the same diameter as Earth, it will appear to be a very small dot when it crosses the sun. Unless you have access to a telescope with a sun filter, I recommend watching the event on-line. There will be many sites that will have telescopic views: Google plus will post many live links in the next 24 hours. The event takes place June 5 and 6 depending where you are on Earth. Note: never look at directly at the transit or the sun. More info here: http://www.transitofvenus.org/