drop function dbo.F_START_OF_WEEK_INCREMENT
go
create function dbo.F_START_OF_WEEK_INCREMENT
(
	@DATE			datetime,
	@WEEK_START_DAY	varchar(10)	= 'sunday',
	@WEEK_INCREMENT	int		= 0 
)
/*
Find the fisrt date on or before @DATE + @WEEK_INCREMENT
that matches day of week of @WEEK_START_DAY.
*/
returns		datetime
as
begin

select @DATE = dateadd(day,@WEEK_INCREMENT*7,@DATE)

declare @WEEK_START_DAY_NUM int

select @WEEK_START_DAY_NUM =
	case lower(@WEEK_START_DAY)
	when 'sunday'		then 1
	when 'monday'		then 2
	when 'tuesday'		then 3
	when 'wednesday'	then 4
	when 'thursday'	then 5
	when 'friday'		then 6
	when 'saturday'	then 7
	end

declare	 @START_OF_WEEK_DATE	datetime
declare	 @FIRST_BOW			datetime

-- Check for valid day of week
if @WEEK_START_DAY_NUM between 1 and 7
	begin
	-- Find first day on or after 1753/1/1 (-53690)
	-- matching day of week of @WEEK_START_DAY_NUM
	-- 1753/1/1 is earliest possible SQL Server date.
	select @FIRST_BOW = convert(datetime,-53690+((@WEEK_START_DAY_NUM+5)%7))
	-- Verify beginning of week not before 1753/1/1
	if @DATE >= @FIRST_BOW
		begin
		select @START_OF_WEEK_DATE = 
		dateadd(dd,(datediff(dd,@FIRST_BOW,@DATE)/7)*7,@FIRST_BOW)
		end
	end

return @START_OF_WEEK_DATE

end
go
set nocount on
declare @DATE			datetime
declare @WEEK_INCREMENT	int

select @DATE			= '2006/1/7'
select @WEEK_INCREMENT	= 1

select  [Def]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,default,default)
select  [Sun]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'sunday',@WEEK_INCREMENT)
select  [Mon]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'monday',@WEEK_INCREMENT)
select  [Tue]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'tuesday',@WEEK_INCREMENT)
select  [Wed]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'wednesday',@WEEK_INCREMENT)
select  [Thu]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'thursday',@WEEK_INCREMENT)
select  [Fri]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'friday',@WEEK_INCREMENT)
select  [Sat]=dbo.F_START_OF_WEEK_INCREMENT(@DATE,'saturday',@WEEK_INCREMENT)


Results:

Def
------------------------
2006-01-01 00:00:00.000

Sun
------------------------
2006-01-08 00:00:00.000

Mon
------------------------
2006-01-09 00:00:00.000

Tue
------------------------
2006-01-10 00:00:00.000

Wed
------------------------
2006-01-11 00:00:00.000

Thu
------------------------
2006-01-12 00:00:00.000

Fri
------------------------
2006-01-13 00:00:00.000

Sat
------------------------
2006-01-14 00:00:00.000