Rare transition of Venus across the sun, for the western hemisphere occuring during sunset on June 5, 2012. Unless you can live another 105.5 years, this will be the last time in your lifetime you can experience this event.
Find out where to see a rare astronomical event that won't recur for more than a century, in this SPACE.com infographic.
Source: SPACE.com: All about our solar system, outer space and exploration
It is kindof a special kind of solar eclipse, just not of the moon, instead of Venus that is far enough away that it will just be a small dot crossing in front of the surface of the sun.
You will be able to see it without magnification from a telecsope of binoculars, but you must use eye protection, such as the welding goggles I described in the May 20 2012 solar eclipse page.
It is so rare because there are only 2 planets that can possibly get between Earth and the sun: Mercury and Venus. Mercury is smaller and farther, so it is more difficult to see but transits the sun more often because it makes a complete revolution around the sun every 88 Earth days. Venus takes 225 Earth days.
This video explains it and why it used to be important to observe the rare Venus transition. (To find out the size of the solar system)

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Some information from NASA
Find out about the planet Venus' dramatic trip across the face of the sun in June 2012 in this SPACE.com infographic.
Source SPACE.com: All about our solar system, outer space and exploration