Grouping by week

I'm using 

SELECT EXTRACT(WEEK FROM trans_date), SUM(tran_amount) ... GROUP BY
trans_date 

and it is being used to group sales results by week. It works really well. 

What I'm wondering is if I can shift the week from a Mon-Sun
articulation(default with Postgre) to a Sun-Sat sequence. I need it that way
in order to comply with a legacy stats system.

Thanks,
Caleb