JCT - IVD dates are these week days or calendar days?

Is the due date (following IVD) counted as week days or calendar days and if calendar days what happens if the due date falls on a weekend?

You need to check your Sub-Contract Particulars but assuming they follow the standard JCT approach the Interim Valuation Date is a fixed date in each month and if that date falls on a weekend or public holiday, is the nearest Business Day in that month.