350.org was founded by U.S. author Bill McKibben, who wrote one of the first books on global warming for the general public, and a team of university friends.
The number 350 signifies the safe upper limit of carbon dioxide in the atmosphere. Currently the number is 387, but can be changed with some simple choices.
In 2007 a campaign called Step It Up organized over 2,000 rallies at iconic places in all 50 states. Their creative actions - from skiers descending a melting glacier to divers hosting an underwater action - helped convince many political leaders, including then Senator Barack Obama, to adopt our common call to action: cutting carbon 80% by 2050.
This year, the day is October 24, Saturday.