BussinesDaysToCalendarDays

ALTER FUNCTION [dbo].[BussinesDaysToCalendarDays]
(
    @BussinesDays INT,
    @WeekDayToStart INT,
    @BussinesWeekStart INT,
    @BussinesWeekEnd INT    
)
RETURNS TABLE
AS 
RETURN
(
    WITH Digits(Digit) AS ( SELECT 0 UNION SELECT 1 UNION SELECT 2 UNION 
                            SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION
                            SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION 
                            SELECT 9 ),
     Numbers(Number) AS ( SELECT Centuries.Digit * 100 + Tents.Digit * 10 + Units.digit 
     FROM Digits Units 
     CROSS JOIN Digits Tents 
     CROSS JOIN Digits Centuries 
     ) -- This is just a tally table from 0 to 999
     
     
    SELECT CalendarDays = MAX([CalendarDay]) FROM 
    (
        SELECT TOP (@BussinesDays)
             [CalendarDay] = d.Number + 1 ,
             [WeekDay],
             IsBussinesDay = CASE WHEN [WeekDay] BETWEEN @BussinesWeekStart AND @BussinesWeekEnd THEN 1 ELSE 0 END
        FROM Numbers d 
        CROSS APPLY (SELECT [WeekDay] = ((@WeekDayToStart -1 + d.Number ) % 7) + 1 ) wd
        WHERE Number BETWEEN 0 AND 365 AND [WeekDay] BETWEEN @BussinesWeekStart AND @BussinesWeekEnd
        ORDER BY d.Number 
    ) BussinesDays
)