I assume you are fetching MTD data depending on selected date and you want to form groups of 7 days starting from the end date.
create variables as below:
v_SelectedDate =ToDate(UserResponse("<Date Prompt Text>");"INPUT_DATE_TIME")
v_Week ="Week "+Ceil((DaysBetween ([Date Object];[v_SelectedDate])+1)/7)
Now, use v_Week object in your table.