SQL Function that returns working day of month by index (returns only day number).

WORKING_DAY_OF_MONTH.sql

CREATE FUNCTION WORKING_DAY_OF_MONTH
(
    @DayIndex INT,      -- Searched day index
    @Month INT,         -- Month
    @Year INT           -- Year
)
RETURNS INT AS 
BEGIN
    DECLARE @Index INT = 1
    DECLARE @DaysFound INT = 0

    WHILE (@DaysFound != @DayIndex)
    BEGIN
        IF(ISDATE(CONCAT(@Year, '-', @Month, '-', @Index)) = 0)
        BEGIN
            RETURN @Index-1
        END

        IF(DATEPART(WEEKDAY, DATEFROMPARTS(@Year, @Month, @Index)) IN (1,2,3,4,5))
        BEGIN
            SET @DaysFound = @DaysFound + 1
        END

        SET @Index = @Index + 1
    END

    RETURN @Index
END