To simplify it for you, they do it during business hours so that they, like us, can enjoy their nights and weekends too.
Besides, for something like a stress test, I'd imagine they want to keep an eye on the stress that is on the servers in real time. Watching something as it happens is usually more helpful than trying to look over four hours worth of data the next day.
Not only that but they are more than likely getting to the point where most employees will be putting in a TON o overtime to make sure GW2 gets finished. But that will probably happen once we do get a release date.