MRP and lot effectivity

Would anyone know how MRP calculates lot effectivity when the effective date falls on a non working day?My initial tests indicate if the effective date falls on a non work day, MRP and time series considers the lot available on the previous work day, rather than the next work day.  It would seem to make sense if the calculation would move the availability to the next work day.I am not clear if there is a processing option or some other user controlled setting that I am not aware?