You can use a pivot table and drag the dates header to the row and the sales
to the data,
click finish, the right click in the table and select group, then  clear
month and select days and then
in the number of days select 7 and click OK
Could look like something like

     Sum of Sales
     Dates Total
     11/1/2007 - 11/7/2007 20
     11/8/2007 - 11/14/2007 22
     11/15/2007 - 11/21/2007 20
     11/22/2007 - 11/28/2007 22
     11/29/2007 - 11/30/2007 6
     Grand Total 90