Select your current payday first. Then you can pick ZA Payday within the 7 days before your current payday. You can select 31st if your current payday is the last day / working day of the month. If 29th - 31st is not available that month, ZA Payday will be moved up to the last day of that month.
For example, if your current payday is on the 28th, you can choose your ZA Payday from 21st to 27th.