Its different days/weeks in different locations.
But the entire month is Bike Month.
If you haven't already used a bike for transportation this month, its not too late. It doesn't have to be work, and it doesn't have to be one a particular day.
I spent my local BTWD supervising a crew of volunteers at the East Bay Area's largest energizer station at Oakland City Hall, where we served free pancakes, gave out over 750 goodie bags, valet parked 250 bikes, provided free tune-ups to several dozen, and signed up almost a hundred new members to the local political bicycle advocacy group.
14 hour day, but totally worth it.
Also, my second ever post as a professional blogger is on Bike To Work Day / Month:
http://ecomodder.com/blog/infinity-miles-per-gallon