I think these notes are BAD. It’s a communal space, and they are basically saying ‘we are booking it, fuck off’.
Presuming it’s the only outdoor space, what if you wanted to catch some sun at the weekend? You’re going to be unable to go up there for large chunks of the entire weekend, which sucks. I’d be worried that this set a precedent of people just ‘booking it out’ for themselves . A far better way to handle it would be a note through each door saying that you were hoping to have a celebration on the roof but wanted to check first that this was ok and didn’t cut across anybody’s plans- and providing a mobile number to call/ text if anybody wanted to discuss. If you are the party host then the risk with this approach is that somebody may say that they did have plans to use to the space, but at least then you have a opened a dialogue to discuss and resolve this in a fair fashion. 3pm - 7pm is WAY OTT for a kid’s bday party btw. two or three hours is the max.