"One of the rarest astronomical phenomena, a transit of Venus, will occur on the evening of Tuesday, June 5. Similar to a solar eclipse, Venus will move across the face of the sun and block light from the sun to Earth. The planet's transit will take about six and a half hours to travel across the face of the sun.
This is your last chance to see a Venus transit until 2117, so make plans to watch this rare and beautiful event."
More info-> NASA - Web Chats