They will be starting from Freedom Plaza, Pennsylvania Avenue between 13th and 14th. Apparently, they have gotten permission to route traffic around there. The closest metro station is Federal Triangle.
The map will be available for download (as a .pdf) late on Friday afternoon.
If anybody is planning to gather afterwards and while away the late afternoon, The Elephant and Castle pub at Pennsylvania and 12th is a decent watering hole. (They do have food, but I have never been there for anything other than drinks.)