The transit of Venus is when the orbital path of Venus takes it across the Sun, viewable from Earth. After this event, which spans today and tomorrow morning (depending on which part of the world you live in), no one alive is likely to see it again.
This happens every century, in a pair eight years apart and the last time this happened was in 2004. Venus will appear as a dark spot that gradually crosses the face of the Sun. This bookend of the 2004-2012 pair should be a better opportunity for astronomers to practise looking for exoplanets, as the Sun is in an active phase of an 11-year cycle that will provide better information when detecting Venus’s signal.
For amateur observers though, the visibility of the full transit will depend on which part of the world you’re viewing from. Venus will start to cross the Sun at 2209 UTC (Universal Time Coordination) on 5 June and finish on 6 June, 0449 . So for Europe, that means getting up early on 6 June to watch the end of the transit, while the US can watch the start of the transit on 5 June, just before sunset. Other parts of the world, including Japan, most of Australia, the Pacific islands, Russia, parts of Canada, the Arctic and Alaska, will be able to see the entire transit. But parts of Africa, most of South America and the Antarctic (being in perpetual darkness at this time of year) won’t see any of it. Iceland sits entirely in a relatively small area of viewability dubbed ‘region X’, where people can see the beginning and end of the transit, but the Sun sets during its peak.
Successful viewing is dependent on local weather conditions, of course. But if the skies in your area are clear at the time, the same rules apply for the transit of Venus as for viewing a solar eclipse: do not stare directly at the Sun. Direct, strong UV exposure for any period of time can damage the retina causing temporary or even permanent blindness. The best way to view the transit is to use a solar filter of some kind, like a #14 welder’s glass, or a telescope with a filter specifically designed for viewing the Sun. There are also projection techniques you can use with a pair of binoculars or a classic pinhole box.