working day = day that you work, and is a local expression business day = recognized business days for the whole country i.e M-F excluding bank holidays. If I work on Sunday then Sunday is a working day for me, but it is never a business day!

'working days' are the days you go to work and get paid for them except for the one or two days off you take per week. 'Business days' are specific days, not regular ones ,where business transactions are taking place .

a business day is a day when you do selling and buying... to make a transaction whereas a working day is a day when you work regardless if it a day of rest or not. If you are a person working in a company that does business say a restaurant on Sunday, then Sunday is a business day for the restaurant and you working in the restaurant, Sunday is a working day for you even though Sunday may be a non-working day for most people.