"On Saturday April 6th we will once again celebrate World Pillow Fight Day with a massive pillow fight on Trafalgar Square. It's the most fun you can have on a city square and on this day, it happens in hundreds of cities around the world.

There are only 2 rules:
Don't hit anyone with a camera and don't hit anyone without a pillow..."

http://2013.pillowfightday.com/events/2221/london