We are struggling with the "prompt payment" requirement as well. Cut-off for our business day, which is prominently posted, is 3:00 pm. We have a process to capture those transactions that occur between our cut-off time and when our bank closes each day, but are unsure of how to treat a payment received on a Saturday.
On Saturday's we are processing all transactions on Monday's business day (our 3:00 cut-off is disclosed to customers). Do we have to back-date all payments received on a Saturday to that actual date or can we let the payment process on Monday's business?