If you're heading out on Sunday and don't know what to wear, stop thinking. January 8th is the 16th annual No Pants Subway Ride, the one day each year when the metro system is clothing-optional.

Getty Images/New York

Organized by Improv Everywhere, the first No Pants Subway Ride was in 2002 and featured seven pantless participants. According to the group, tens of thousands of people in more than 60 cities and 25 countries left their pants at home last year.

Getty Images/New York

What are the rules? According to Improv Everywhere, "Random passengers board a subway car at separate stops in the middle of winter without pants. The participants behave as if they do not know each other, and they all wear winter coats, hats, scarves, and gloves. The only unusual thing is their lack of pants."

HENRY ROMERO/Reuters/Landov/Mexico

Though the event began in New York, people in cities from Portland to Paris to Prague are planning to participate this year. See the complete list.

ALY SONG/Reuters/Landov/China

In previous years, it has even spread to China—though it doesn't look like anyone told this guy's fellow passengers.

JOHN ANGELILLO/UPI/Landov/New York

GLEB GARANICH/Reuters/Landov/Kiev, Ukraine

BIMAL GAUTAM/Barcroft Media/Landov/London

FABRIZIO BENSCH/Reuters/Landov/Berlin

Getty Images/Berlin

Getty Images/New York

Chad Nicholson for ImprovEverywhere.com

Chad Nicholson for ImprovEverywhere.com

Here's a recap of 2016's No Pants Subway Ride festivities:

Portions of this post appeared in 2015.