Scheduled for 18th December, 1 PM, at the concourse of Grand Central Station.
"Where? Why?
These are questions that will be answered later. Just remember this: Dec 18th, 1PM.
Plan on taking your camera out for a day of photography that won't ever be forgotten, with a flash mob photo session that will even make the MTA board want to be there with cameras. It'll be one of those 'only in new york' things you've been hearing about...
What exactly is going to happen?
You'll need to be there to see for yourself. Rest assured, it will be nothing dangerous or illegal, and it will be something no one will soon forget."
Additional info from these links: