Try the this method of python,, Isoweekday() is a method of the DateTime class that tells the day of the given date. It returns an integer that corresponds to a particular day.

It seems like since 2013 and Odoo v6 -  this problems still wasn't fixed?

You need to modify this method '_get_number_of_days', defined in hr.holidays object by using some datetime attributes that will exclude weekend from number of days.

Thanks, Priyesh Solanki