Calculating working days per month

I have a Rolling Calendar table with a number of columns as below:

I have managed to get as far as adding a column which calculates a work day or not (Workday column), but now I would like to add a column which calculates the number of working days per month (similar to the Days in Month column as per the screenshot).  Is there any way to do this in this table by adding a calculated column.  

Alternatively, I would be open to adding another table, which has a one row representing each month and the number of working days in each.

I need to use this "Total of working days" in a number of calculations across various other tables, with data that is collated by month.   

Help please!