Unfortunately none of the Time intelligence functions in DAX supports the interval “week” in order to get the same week last year.
In my case the data comes by week and I use the monday as the date key for the sales.
So the 3-1-2011 is a Monday
Sales Amount LY:=CALCULATE(SUM(FactStoreSales[SalesAmount]), PARALLELPERIOD(DateTime[Date],-1,year))
This means the this DAX expression will point to 3-1-2010 – but week 1 2010 actually starts with 4-1-2010.
In order to solve that I changed my expression to
Sales Amout LY:=CALCULATE(SUM(FactStoreSales[SalesAmount]), DATEADD(DateTime[Date],-364,DAY))
That did the trick J
Be aware that this means that years following years with 53 weeks – week 1 will be matched against week 2 in the previous year.