Based on the original post, I'm thinking you want something like this:

declare @TestDate date = '20120101',

@DaysToAdd int = 8;

select

@TestDate,

@DaysToAdd,

dateadd(dd, @DaysToAdd, @TestDate),

datepart(wk,dateadd(dd, @DaysToAdd, @TestDate)),

dateadd(dd, datepart(wk,dateadd(dd, @DaysToAdd, @TestDate)) - 1, dateadd(dd, @DaysToAdd, @TestDate));