use two custom functions, like these:
Then you can just calculate it as:
WorkingDays ( get(currentDate) ; LastDayOfMonth ; holidayList )
Are we to infer that each record in your table corresponds with a unique selling day?
If so, what happens if a user creates a record and then later deletes it? Should your "counter" include the deleted day, or should it re-calculate based on other days in the table?
You can also get last day of month by calculation, instead of asking the user.
Date ( 3 ; 0 ; 2016 ) returns 2/29/2016, for example.
You should change function name as "WorkingDays" and remove 4th paramter.