You can definitely make the marshmallows tonight and store them until Monday. They may not taste as fresh as they would if they were eaten tomorrow, but they definitely will not harden. The caramel will stay soft, and the cereal mixture will stay crunchy. You can store them in an airtight container at room temperature. Try storing them with a soft piece of bread for added moisture. Good luck! You’ll have to let me know how they turn out!