Earth Hour is the world’s largest global climate change initiative. At 8:30 PM local time on Saturday 26th March 2011, lights are turned off around the world.
It has now swept across Oceania, Asia and Europe.
Many famous landmarks have turned off their lights for an hour. Hundreds of millions of people have joined.
Earth Hour takes place at the same time all over the world, and rolls like a wave from Fiji. It was first held in 2007 and quickly became a very popular event.
Worldwide, 128 countries participated in 2010.
The timing is chosen for cosmic reasons. It is a time when it is reasonably dark across the earth so that it is suitable for all nations.
Photos of Earth Hour:

China, Beijing - Bird's Nest

France, Paris - The Eiffel Tower

Norway, Oslo - The Opera

USA, San Francisco - The Golden bridge
From The Earth Hour’s website:
Earth Hour started in 2007 in Sydney, Australia when 2.2 million individuals and more than 2,000 businesses turned their lights off for one hour to take a stand against climate change.
Only a year later and Earth Hour had become a global sustainability movement with more than 50 million people across 35 countries participating.
In March 2009, hundreds of millions of people took part in the third Earth Hour.
On Saturday 27 March, Earth Hour 2010 became the biggest Earth Hour ever. A record 128 countries and territories joined the global display of climate action.