Monday is a business day. If I have a shipment whose current status is "next business day" today, I'd expect a Monday delivery, barring shipment issues.
Monday to Tuesday is one business day. Saturday to Tuesday is one business day fror most companies because its technically Mon to Tue - the days things actually move. Just because it ships on Saturday doesn't mean it's going anywhere on the weekend. Some big time companies, like Amazon, with their own hub, can call Sat to Monday one business day. Apple is huge too, but I don't know if they have their own hub.