I recently came across an assignment for Tableau that required me to find the total number of business days in each month over several years. When originally presented with the data, all I had to go off of was the order date for each order that was given to me. This left me extremely confused. I had no idea how to create a calculation that analyzes the days in each month based off of order dates.
After doing a little research I was able to find an excellent post that explains step by step how to go about doing this. You may need to download a couple extra worksheets (for example when counting holidays), but the author does an excellent job of showing what calculated fields are required and also provides pictures to give assistance.
You can find the full article here.