Declare @tbl_Data Table(Dates Date,WorkDayCount Int)

Insert Into @tbl_Data Select '2012-08-25',1
Insert Into @tbl_Data Select '2012-08-26',1
Insert Into @tbl_Data Select '2012-08-27',1
Insert Into @tbl_Data Select '2012-08-28',1
Insert Into @tbl_Data Select '2012-08-29',1
Insert Into @tbl_Data Select '2012-08-30',1
Insert Into @tbl_Data Select '2012-08-31',0
Insert Into @tbl_Data Select '2012-09-01',1

Declare @InputData Date = '2012-09-01'

;WITH CTE AS
(
    SELECT Sequence = ROW_NUMBER() OVER(ORDER BY Dates DESC),*
    FROM @tbl_Data
    WHERE  Dates < @InputData
)
,CTE2 AS
(
    SELECT
        Sequence = ROW_NUMBER() OVER(ORDER BY Dates DESC)
        ,Dates
        ,WorkDayCount
    FROM
    (
        SELECT * FROM CTE WHERE Sequence  = ((SELECT TOP 1 Sequence FROM CTE WHERE WorkDayCount = 1)-1)
        UNION
        SELECT TOP 2 * FROM CTE WHERE WorkDayCount = 1
    )X
)
SELECT TOP 1
            Date = CASE WHEN  Sequence = 1 AND  WorkDayCount = 0 THEN (SELECT Dates FROM CTE2 WHERE  Sequence = 3)
                    WHEN  Sequence = 1 AND  WorkDayCount = 1 THEN (SELECT Dates FROM CTE2 WHERE  Sequence = 2)
            END
FROM CTE2

SELECT MIN(DATE_COL)
FROM  (
        SELECT  TOP 2  DATE_COL
        FROM TBL_DATE
        WHERE DATE_COL<='2012-09-01'
        AND WORKDAYCOUNT =1)A