Ploompf!! Pillow Fights Erupt Across The Globe

Hide caption Pillows fly near before the Elizabeth Bridge on the Danube in Budapest, Hungary, on Saturday, International Pillow Fight Day. Previous Next Janos Marjai/EPA via Landov

Hide caption In London, the pillow fight was in Trafalgar Square. Previous Next Olivia Harris/Reuters via Landov

Hide caption In Hong Kong, they fought with pillows a week early. Previous Next Jerome Favre/EPA via Landov

Hide caption Germans fought in front of the Brandenburg Gate in Berlin. Previous Next Adam Berry/AP

Hide caption After the fight, German municipal workers had to clean up the feathers. Previous Next Thomas Peter/Reuters via Landov

Hide caption In Shanghai last Saturday, police broke up the fight. Previous Next Carlos Barria/Reuters via Landov

Hide caption And in Taiwan, International Pillow Fight Day apparently had a sponsor. Previous Next Shengfa Lin/Reuters via Landov 1 of 7 i View slideshow

If it's not already marked on your calendar, here's your warning: Today is International Pillow Fight Day. Cities around the world are taking the holiday seriously — as serious as a pillow fight can be, anyway.

Organized through Facebook and other forms of social media, the flashmob fights are often surprises for an unsuspecting public caught on a field of sudden battle. In Shanghai, as MSNBC.com reports, the cushy conflict was hastily broken up by police after just five minutes.

If you are unfamiliar or unprepared, don't worry; Pillowfightday.com has a handy guide plus a list of cities that are participating. "Never ask permission" is one of the tips the site offers.

There are rules, however, and WHYY's Newsworks site helpfully supplies some important ones: