It's not as if M.S. haven't told everybidy already that the date is the STAR T of the roll-out.
I doubt is even they have the server and bandwidth capacity to roll it out in one day.
Cue the 'Took hours', 'why haven't I got it yet' 'M$ is shit as mine wasn't first' and "M$ has killed the internet" comments around the world. later on this month.
Some of us may prefer to defer it for a while until W10 SP2 comes out. (or whatever they will call it -- W10.2?)