What started as a small prank in New York City has grown into an international stunt, with dozens of cities around the world participating each year.
The idea is silly but simple: passengers board an underground train in the middle of winter without trousers (or pants, as Americans say). The participants behave as if they do not know each other, and they all wear winter coats, hats, scarves, and gloves.
So far more than 500 Vienna residents have pledged to take part. They will meet at Karlsplatz U-Bahn station at 3pm, where they will all take their trousers off together.
The weather forecast predicts that temperatures will be around 3C, so we will be impressed if that many people do dare to bare their legs.
Participants are asked to wear sensible underwear, and to avoid offending other passengers. “If you don't wear any underwear you will be breaking the law, and that is not our goal!”, the organisers warn.