How to autofill Excel cells by week?

If you just want to drag down, put 07/01/2011 in A1, and 07/08/2011 in A2, select them both and drag the grippie down.

I post this as an alternative solution to jonsca's solution. In Cell A1, you can put your starting date. Then, in cell A2, put the formula =A1+7. Then, just click and drag down cell A2 as far as you need. If you want to change the starting date, just change the date in cell A1.

Again, the other way to do this would be as jonsca mentioned, by using Excel's "smart" auto fill feature. It detects the difference between two cells and auto-increments it using the drag-to-fill-down feature. The only advantage to the method I posted would be if you need to change the starting date.