Do you want to know what the next working day is for a given date ? You will need the calendar to check this against. Use function module BKK_GET_NEXT_WORKDAY. The factory calendar you would need for this is Plant master data information, pick it up from T001W-FABKL.
Factory calendars describe which days are working days for a certain factory (plant). It describes whether the Saturday is a working day and also holds public holidays or big maintenance events. If you want to determine when something is ready to be shipped, and you know the estimated number of working days required is e.g. 5 days, the end date calculation should take the factoriy calendar into account.
Use the BKK_GET_NEXT_WORKDAY module to determine what the first working day on or after a given date is. Use the BKK_ADD_WORKINGDAY module to calculate a number of days from a given date. You will notice that 20 working days becomes something close to a month.