Well, this is where a great deal of controversy originates (and many threads):
The indisputable fact is that opening the drapes was a key move in the planned assassination of Michael--FFC leads us right to it when he has Kay remark on the fact that the drapes are open, and the shooting begins almost instantly. I believe a majority of people here think Fredo opened the drapes, if for no other reason than he was more likely than, say, Johnny Ola's men to get into the bedroom without raising suspicion. Jimmy Buffer's point to the contrary isn't invalid, but Fredo remains the most likely culprit.

After that, the general consensus falls apart. Some people believe that Fredo didn't intend for Michael to be killed--he thought he was facilitating a kidnapping, and was dumb enough to fall for that line of BS. Others (full disclosure: including me) believe that he had to know it was gonna be a hit: why else would he open the drapes--so the gunmen could peeping-tom Kay as she undressed? And that controversy leads to another: did Fredo deserve to die? I don't want to stir up any of those again. Just use the "search" function if you want to follow up.