How to find the previous weekdays data for the selected week

[#1]

Hi,

how to find the week over week data

SalesTable:

Date, Sales

1-1-2012, 10

2-1-2012,  20

8-1-2012, 30

9-1-2012, 10

Calendar:

Date, Week, Weekday

1-1-2012, 2012W01, Mon

2-1-2012,  2012W01, Tue

8-1-2012, 2012W02, Mon

9-1-2012, 2012W02, Tue

Now I want to analyse the sales growth by each day for the selected month

for the Week=2012W02

Day   Growth

Mon  20 (30-10)

Tue   -10 (10-20)