Copy Interval Of Seven Rows Continuously?

I am currently trying to sum up values every day of the week. (salaries )

That means I have a sheet called Salary where I have the entire year mapped out pr. day. Like you see underneath:

Date
30/12/13 0
31/12/13 0
01/01/130
02/01/130
03/01/130
04/01/130
05/01/130
etc.

In another sheet called Overview, I have another column divided into weeks, like this:

Date
30/12/12 0
06/01/13 0
13/01/13 0
20/01/13 0
27/01/13 0

Now what I need to do is, take the 7 days in a week in the first sheet, and summarize it in the first week in the overview sheet. Now that is simple the first time, =SUM(Salary!C1:C7)

But how do I drag down and automize it so that the next row becomes =SUM(Salary!C8:C14) instead of =SUM(Salary!C2:C8)?

So basically I need to autofill with intervals of 7 (the 7 days in the week) on every row in my Overview sheet.