From www.earthhour.org:

In 2008, 24 global cities will participate in Earth Hour at 8pm on March 29. Earth Hour is the highlight of a major campaign to encourage businesses, communities and individuals to take the simple steps needed to cut their emissions on an ongoing basis. It is about simple changes that will collectively make a difference – from businesses turning off their lights when their offices are empty, to households turning off appliances rather than leaving them on standby.

earth hour