The next transit of the Planet of Venus across the face of the sum will occur over Tuesday-Wednesday, June 5-6. In astronomy, a transit occurs when a smaller celestial body passes across the face of a larger celestial body from the point of view of the observer.
From Earth, there are only two regularly occurring transits of celestial bodies across the sun -- one by the planet Mercury and the other by the planet Venus. In truth, a third celestial body crosses the sun, but that is the moon and it completely covers the sun when the alignment is right. But because these events cover so much of the sun, and in some cases all of the sun, these occurrences are more precisely called occultations (coverings) rather than transits (crossings).
For reasons which we will get into later, transits of Venus occur in pairs that are just a few years apart. The last transit occurred June 8, 2004. The second of the pair occurs over June 5/6. After that, the next pair will in December 2117 and December 2125.
For more on the June 2012 Transit of Venus, visit these links.
2012 Transit of Venus, NASA/Goddard Space Flight Center: eclipse.gsfc.nasa.gov/OH/transit12.html
Transit of Venus, Sun-Earth Day 2012, NASA: sunearthday.nasa.gov/transitofvenus/