Grouping data in a pivot table

I have created a pivot table which contains each day of the year from 1/1/13 to 9/30/13. Column A contains each date and column B contains the total number of sales for that date. I have 3 such sheets, each representing sales of a different product. I need to make a line graph, with each product being a different colored line on the graph. My problem is I need the sales dates grouped together by week, not individual days. Is there an easy way to create this weekly grouping? In other words, I need to figure out how to make column A the week (1/1/13-1/7/13; 1/8/13-1/14/13 etc...) and column B the total for that week, as opposed to having individual days represented.