If we assume Fredo 'knew' there was going to be a hit on Michael, then we have to assume he knew it would be THAT night. It's possible that while they were blowing smoke at him with this 'negotiations' farce, the at-home assassination was being planned behind his back. All they needed was a way to infiltrate the compound, and even if Fredo assisted with that in some way, he just doesn't seem capable of being involved in Michael's murder.

As for who opened those drapes, the answer still stands that there is NO logical answer to that. Because whoever opened them would have to have done it while Kay was lying there, which would take quite a bit of deftness that a Fredo Corleone was incapable of. For the drapes to have been opened earlier in the day, amidst all the activity of both party and business...would have been virtually impossible. And even if someone managed it, they would have been immediately closed by Kay upon her entering the room to get ready for bed.

There is no logical answer to the opening of the drapes. Fredo did not know there was going to be a hit on Michael.

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