I just posted this in another thread, but I know who opened the drapes.
It was ANTHONY. He is the only person we know for sure who had access to the bedroom before the shooting because he put that drawing on Mikes pillow. Perhaps Fredo suggested that while he was in there he open the drapes to "surprise" his dad.
This explains why Anthony was so screwed up as a kid.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
| 
