I find it hard to believe parents who wouldn't know that their kid has been skipping school and possess a weapon such as is used in these crimes would give a flip what their teenager is doing on the weekend or any other time.
Look at the parents of the Columbine shooters. They didn't even know that bombs were being constructed in their garage. I doubt they would blink an eye if their son and a friend were gone for a few hours on a Saturday or Sunday.
Don't forget that neighbors would be more likely to notice them on the weekend too. But as I said earlier, the fact that he doesn't strike during the weekend doesn't mean anything. It could be for all kinds of different reasons that could support conflicting theories. For instance: the guy's company is closed over the weekend/ traffic too light/ neighbors at home and more able to notice something/ Even God rested on the sabbath. etc... etc....
Besides, we don't even know for sure that the Sept 14 shooting wasn't the first of the series, And it was on a Saturday night.
So as far as I'm concerned, the no shooting over the weekend doesn't support nor disapprove any of the many theories advanced here.