Working days (or business days) commonly refer to the days that most South Africans are at work, namely Monday to Friday.
Working days excludes government-recognised public holidays such as New Years Day, Human Rights Day, Good Friday, etc.
NOTE: SARS has its own definition of working days, which includes a holiday from 15 December to 15 January for all of their staff. If you are waiting for any assessments or dispute outcomes from them, please take into account the extra month delay over this time.