My best guess, and it is only a guess is April 29,30, May 1(Fri, Sat, Sun). That being said, usually folks start showing up the Friday before. In this case that is April 22.
Usually there is lots of bow staves available along with other venders and the workshop is open to anyone until Fri(29), Sat(30) and Sun(1). On those 3 days the bow workshop is only available to the folks that sign up for the Selfbow Challenge. I believe after 6pm on that weekend the shop is open to anyone and that goes on until the last man standing.
As soon as Pappy posts the official announcement I'll post it here on the Bowyers Bench and on PowWow.