This has received rather more publicity this year - I've heard it mentioned three times on the BBC radio News this morning, including a short interview.
There is an article on their website, too.
In Europe the Eiffel Tower, Big Ben and Edinburgh Castle will be in darkness for one hour.
A symbol, indeed, but an important one.
Though there are dedicated environmentalists such as George Marshall, founder of the Climate Information Outreach Network , and the blog who argues against this action, as ineffective and likely to resonate badly with the unconvinced, to whom darkness is a Bad Thing, associated with death, decay and privation.
And there are plenty of 'rebels' posting on the comments threads, pledging to leave all their appliances ON.