0 of 0

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:

Bring a soft pillow with a clean cover, sans feathers if possible.No foreign objects in the cases: pillows only.Be discrete prior to the event and keep all pillows secured.Only hit others who also have a pillow.Please, no beanbag chairs or body pillows.Copyright 2016 NPR. To see more, visit http://www.npr.org/.