Using The SUM Function To Record Weekly Sales

I would like to have a set of cells that add up all the sales within a given week. I know how to do this simply for one week, but how do I get Excel to automatically take this function and create the rest for future weeks?

After entering the SUM function in one cell, I click and drag on the box to try to get Excel to correctly input the functions in the next cells (like how Excel will correctly input the next date, week, or month). But Excel doesn't do it correctly.