How to group by dates when dates have different criteria?

I have a report based on dates and working days. The working days are used to compute productivity. The number of working days changes each month, especially since we don't follow a usual holiday schedule. So for July and August of this year I have working days of 20 and 23 which I have put into an excel sheet that i've connected to my report (we don't keep track of working days in the database). Some in my database I have Jul-16 and Aug-16.

The only dates I have in the regular database are random dates. 7-1, 7-5, etc. I've grouped them by month and have a productivity formula in the group footer. How do associate the grouped month from the database based on days being in the same month with the working days in the excel sheet?

So how does that productivity formula use 20 days in July and 23 days in August?

Thanks