Increase Reference Rows by 7 as I Drag Down

I am trying to create a rolling 28 days summary of some data. So starting at Week4, I have the following formula:

Week4 =median('sheet1'!V2:V29)
Week5 =median('sheet1'!V9:V36)
Week6 =median('sheet1'!V16:V43)

As you can see, I want the row references to increase by 7 each time I drag down the formula...how can I do that? Do I need to switch to R1C1 referencing?

Thanks,

Adam