Factory Calendar with Time-Key Date Scheduling Indicator

My understanding is that when we create a Maintenance Plan with Scheduling Indicator 'Time - key date' assigned, the Preventive Maintenance order will always be scheduled on the same date of every month. However, our SAP system is requiring entry of a Factory calendar when we choose the Time key-date indicator. Can anyone explain the reason for this behavior? Our requirement is that Preventive Maintenance orders are scheduled as close to the 1st of every month as possible, but we don't want orders to be scheduled on non-working days. So, if the 1st of a month happens to fall on a Sunday, the system will schedule the order for Monday the 2nd instead.