To set up dynamic rollups go to Settings->System and enter values to the setting field.
Create automatic period rollups (enter weeks eg -12,-4,4,12)
You can enter list of number separated by commas to define the rollup weeks.
0 - current week
1 - next week
-1 - previous week
-4 - four previous weeks
So in order to have rollups as in the above example you would enter the following: 0,-1,-4,-12-52,8.
New elements in Week dimension will be created with id-s shift_<value> e.g. shift_-1 (for last week). Since these are non-system elements you can remove them manually. When doing so also make sure you remove them from the above setting or they will be recreated.