If Fredo didn't realize it was going to be a hit (which I believe he didn't)...then what possible reason would he have for opening the drapes to give assasins a clear shot at Michael's bedroom???

Stupid and gullible as he was, even Fredo would've had second thoughts as to a request like that.

If we're going to ponder who opened the drapes, then we also have to ask how Kay could possibly get undressed and into bed without ever realizing they were open. Or, we would have to assume someone snuck in and opened them while she dozed off...neither is too believable.

So the answer is that there is really NO answer as to who or how the drapes got opened. They just got that way, and it's just another 'black hole' in the movie. Because it was something the writer/director probably did not anticipate people asking.

Apple