Try the following:
=(DATEDIFF(DateInterval.Day, CDATE("2016-02-14"), CDATE("2016-02-17")) + 1)
-(DATEDIFF(DateInterval.WeekOfYear, CDATE("2016-02-14"), CDATE("2016-02-17")) * 2)
-(IIF(WeekdayName(DatePart(DateInterval.Weekday,CDATE("2016-02-14"),FirstDayOfWeek.System))="sunday",1,0)
-(IIF(WeekdayName(DatePart(DateInterval.Weekday,CDATE("2016-02-17"),FirstDayOfWeek.System))="saturday",1,0)
))
, 3. StartDate = 2016-02-14 EndDate = 2016-02-21 5.
UPDATE: .
=(DATEDIFF(DateInterval.Day, Parameters!DateFrom.Value, Parameters!ToDate.Value) + 1)
-(DATEDIFF(DateInterval.WeekOfYear, Parameters!DateFrom.Value, Parameters!ToDate.Value) * 1)
-(IIF(WeekdayName(DatePart(DateInterval.Weekday,Parameters!ToDate.Value,FirstDayOfWeek.System))="friday",1,0))
DateFrom ToDate Result
2016-02-12 2016-02-19 6
2016-02-12 2016-02-18 6
2016-02-12 2016-02-15 3
.
, .