For most "personal" products, banks calculate interest on the basis of a 365-day year at a rate of 1/365 of the published rate each day, whether they are charging interest on loans or paying interest on deposits/savings, and this, of course means that the interest is calculated for every day, whether working days or otherwise.
Interestingly, this normally means that they charge 366/365 of the interest rate in a leap year, and pay 366/365 of the rate on deposit accounts.
Certain types of inter-bank funding is calculated on a basis of 1/360 of the rate each day, and where banking products are linked to these rates, the rate to the customer is also on the same basis. Normally, these products are in the business sector.