My maximum chaos effect by attacking on an ordinary weekday while schools are in session is if an external party (China, Russia, terrorists etc) is attacking the US. If the economy is about to collapse and TPTB need a bank holiday, then it would involve a weekend or holiday.
I think the risk of a massive cyber attack is growing while not discounting the possibility of biowar either. Of course it doesn’t have to be an either/or scenario. It can be multiple angles of attack.