This discount includes regular and sale items across their stores. Each sale has it's own promo code, which I posted below. Also, if you spend $50 or more at either of the brands and shipping is included free of charge. This is a pretty simple offer, so I'm walking away without explaining any more.
Click here to go to Gap and use code GAPMAY for the discount.
Click here to go to Banana Republic and use code BRMOMDAY for the discount.