Please use the search function before posting a new question. This particular issue has come up a number of times and be solved in a number of ways - https://community.tableau.com/search.jspa?q=calculate+working+days
Please review those resources first and let us know if you have any specific questions regarding your scenario.
I actually checked all those resources before I posted the question
My issue is Number of days per month say
January 31 - weekends - holidays=20 days
All need to be applied to one field Month.
When sorting weekdays (which is an option you have with dates data type) Tableau consider Sunday to be the first day and Saturday the last day of the week.
You can thus create a filter to exclude the value 1 and 7:
IF DATEPART('weekday',[Date])=1 or DATEPART('weekday',[Date])=7 then "HIDE"
ELSE "SHOW" END
Place that into the filter shelf and select "show"
The result will hide all Saturday and Sundays in your data set.
For more elements about dates, have a look at Tom's links, a lot of useful content listed and available
Hope this helps!