American launches have a lot of trouble with weather and are always changing times. European launches are pretty much on schedule most of the time but can have occasional delays. Russian launches are so robust and well planned that they launch successfully in all conditions – snow, sun, day, night or whatever!
In addition to what Andrea said, all launches are scheduled in a way, that you reach the desired orbit or rendezvous vehicle, also without interfering with anything that’s flying up there already.
So for meeting with ISS, the station has to fly over the launch site just in the timeframe where the launch itself is. If you miss that opportunity due to weather or so, you have to wait until the next launch window. If you just launch spontaneously, you will reach the ISS’s orbit height, but the station might just be somewhere else around the world, far out of reach!
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