It's Monday, Not Saturday

When Jon woke up Saturday morning, he heard someone knocking on his door. Then he heard his mom's voice. "Jon, you better get up and get to school." "School? But it's Saturday. There's no school on Saturday," he countered. "Jon, it's not Saturday. It's Monday," his mom said. "Now get up and get ready." What was happening? Where did his weekend go? Did it happen and he just didn't remember it? Did he somehow travel two days into the future? What was going on?