Here's the national site URL:
http://improveverywhere.com/2013/01/07/no-pants-subway-ride-2013-details-for-new-york/


"All are invited to participate in the 12th Annual No Pants Subway Ride. The event will take place at 3:00 PM on Sunday, January 13. Everything you need to know is in this post. Please read it carefully! (Click here for information for participating in other cities.)

REQUIREMENTS FOR PARTICIPATION:

1) Willing to take pants off on bus [Ann Arbor lacks a subway]
2) Able to keep a straight face about it

DETAILS

When: Sunday, January 13 at 3:00 PM, CC Little, Sharp!"

Ann Arbor's event is linked to the national no pants :-) site at:
http://improveverywhere.com/2013/01/07/global-no-pants-subway-ride-2013/
"Cities participating in the global No Pants Subway Ride:

- Adelaide
- Amsterdam
- Ann Arbor"