The sample you had given is quite unclear, but have a look here:
declare @d datetime
set @d='2013-12-05'
select
DATEADD(dd, -(DATEPART(dw, @d)-1), @d) [WeekStart],
DATEADD(dd, 7-(DATEPART(dw, @d)), @d) [WeekEnd]
It will return 2013-12-01 (Sunday) as week start, and 2013-12-07 (Saturday) as week end. If you want custom week start, play with the constants.
[Update]
Here is the modified version that makes a week to start on Saturday and end on Friday:
select
DATEADD(dd, -(DATEPART(dw, @d)), @d) [WeekStart],
DATEADD(dd, 6-(DATEPART(dw, @d)), @d) [WeekEnd]