Assigning fleets of aircraft to a weekend schedule is more difficult than assigning them to a weekday schedule. The reason is that we must balance two conflicting objectives. We must meet passenger demand that is different from weekday demand. We must also minimize the costs of realigning airport facilities and personnel that we would incur by changing flight patterns too much. To support US Airways' schedule planners in this balancing act, we built a specialized fleet-assignment model and integrated it into a graphical environment for schedule development. The planners use the system to create safe, profitable, and robust flight plans.