Do you need partial months (i.e. this month so far)?
I need all the months (YTD)
The most flexible route in my opinion is to have another data source that is a list of working dates, and use a data blend to pull in the counts as needed. This way you can handle for holidays and other interesting aspects.
If you would like an example, please provide a sample packaged workbook (.twbx) that represents your situation.