This is a pretty rare occurance called the Venus Transit, basically every 105 years or so the planet Venus will move across the face of the sun and will appear as a small black disc (a small spot compared on the sun). Since this is such a rare occurrence, the next time this happens you will most likely be dead, so it is advised to check it out if it is visible in from your country.
In the UK it will end at around sunrise on Wednesday, but will last approximately 6 hours.
However to view this you will need the proper equipment as looking directly at the sun can cause eye injury or blindness, so look for your nearest astronomy club or maybe find a live stream on the net.
Article here:
http://www.bbc.co.uk/news/science-environment-17745366
