A transit of Venus occurs when Venus passes directly between the sun and earth. This alignment is rare, coming in pairs that are eight years apart but separated by over a century.
The most recent transit of Venus was a thrilling sight in 2004. After the June 2012 transit of Venus (the last one in your lifetime), the next such alignment occurs in 2117.
The nearly 7-hour transit begins at 3:09 pm Pacific Daylight Time (22:09 UT) on June 5th. The timing favors observers in the mid-Pacific where the sun is high overhead during the crossing. In the USA, the transit will be at its best around sunset.