Here is another approach with the advantage of restoring the @@DATEFIRST value after your calculation:

DECLARE @dt datetime, @date1st int

-- Save @@DATEFIRST so we can restore it later

SET @date1st = @@DATEFIRST

SET DATEFIRST 7

SET @dt = '10/12/2009'

-- Change to first of the month

SET @dt = DATEADD(d,1-DAY(@dt),@dt)

IF DATEPART(dw,@dt) 7

-- If not already Saturday, add a day until it is Saturday

WHILE DATEPART(dw, @dt) 7

SET @dt = DATEADD(d,1,@dt)

PRINT CONVERT(nvarchar(30), @dt, 101)

-- Restore DATEFIRST

SET DATEFIRST @date1st