The simplest way to achieve this is to modify the "Standard" calendar in MS Project, and set Saturday to be a working day, and the working hours on Saturday to match those used by your company (for example 9am to 1pm). If you then schedule a one day task starting at 9am on Saturday morning, this will finish in the middle of the day on Monday. You can also associate spefici calendars with resources and individual tasks to control how work is scheduled if you need more fine-grained control (for example you can add holidays for individual resources, and so on).