Try adding a new column with the following formula. This would give every
friday date. With this you can match the cash forecast date. Hope this helps.

Sales Forecast New Column (Friday dates)
6/1/2008 =(6-WEEKDAY(A1,1))+A1
6/8/2008 =(6-WEEKDAY(A2,1))+A2
6/15/2008 =(6-WEEKDAY(A3,1))+A3
6/22/2008 =(6-WEEKDAY(A4,1))+A4
6/29/2008 =(6-WEEKDAY(A5,1))+A5