Mark your calendar for March 29, when cities around the world will switch off non-critical lights at 8:00 p.m. for an awareness-raising Earth Hour. At present, 24 cities — with a total population of some 30 million people — plan to participate in the energy-saving symbolism, from Toronto to Tel Aviv, Bangkok to Brisbane, Canberra to Copenhagen, and first Earth Hour participant Sydney to copycat event holder San Francisco. Thousands of individuals and businesses have also signed on to come to the dark side.