WHILE @Iteration != 1000000
    BEGIN
        WHILE @Mth <= 12
            BEGIN
                UPDATE TACC
                SET BUSINESSDATE =
                CASE
                    WHEN DAY([BUSINESSDATE])<13
                    THEN datefromparts(year([BUSINESSDATE]), @Mth, 1)
                    WHEN DAY([BUSINESSDATE]) BETWEEN 13 AND 24
                    THEN datefromparts(year([BUSINESSDATE]), @Mth, 2)
                    ELSE datefromparts(year([BUSINESSDATE]), @Mth, 3)
                END
                SET @Mth = @Mth + 1
                IF @Mth=13
                    BEGIN
                        SET @Mth=1
                    END
            END
        SET @Iteration = @Iteration + 1
    END