On Saturday, March 28, 2009, 8:30 pm local time, cities and groups and individuals around the world are participating in Earth Hour. For one hour, the lights will go out, highlighting issues of climate change and the need for solutions.
Sponsored by the World Wildlife Fund, Earth Hour began in 2007 in Sydney, Australia. In two years it has grown to encompass such well-known landmarks and tourist spots as the Las Vegas Strip, the Golden Gate Bridge, and the Eiffel Tower.
The Earth Hour website has a great FAQ. You can also simply turn out the lights tomorrow night at 8:30 and enjoy the night sky.