Rolling weeks calculation

Hi,

I am trying to set up a spreadsheet where the Average will be always the last 13 weeks (quarter), e.g. current week is 47, so the average would be calculated between week 35 and week 47, next week would be between week 36 and week 48...

The user has to enter the current week and then the data and graphs would show teh results for the 13 weeks taken into consideration the current week..

Any help is appreciated