He already knew which island and on what date he was going to Japan, wow
Well... my Dad's 33rd Infantry Division history describes at length: their training and preparations on Luzon for Operation Olympic, that it was scheduled for November 1, 1945, their relief on learning of the A-bombs, and that after V-J day Olympic was replaced by Operation Black List.
The history does not say how much they knew then, or when they knew it.
Doubtless the same efforts were made to keep precise details secret as had been made for Operation Overlord in Normandy.
Thanks for asking -- I had not previously noticed Operation Black List, which explains why I Corps landed on September 25, near Wakayama & Kobe on Honshu, instead of Olympic's planned Kyushu landings.