The EMP201 declares and pays the PAYE, UIF and SDL for a pay month, and it is due on the 7th of the following month. August payroll is due by 7 September.
The detail that catches people is which way the date moves. If the 7th falls on a weekend or public holiday, the deadline moves earlier, to the last business day before it. Assuming you get the following Monday is a common and entirely avoidable way to be a day late.
Late payment attracts a 10% penalty on the amount due, plus interest running from the due date. The penalty is a flat 10% rather than something that scales with how late you were, so being one day late and being three weeks late cost the same on the penalty line, though the interest differs.
Two related returns often get forgotten alongside it. The interim EMP501 reconciliation closes on 31 October and the annual one on 31 May, and the administrative penalty for a late reconciliation increases each month it stays outstanding. That one genuinely does get worse the longer it is left.