Evidently Borders has extended their 40% off one book coupon to include Monday, Memorial Day, as well. The email I received said "Oops! We meant Monday!" in the subject line, which makes it sound like they screwed up, but who knows, it's probably just a coy marketing ploy.
Whatever - when Borders bats its eyelashes and throws a 40% off discount my way, I'm not one to ignore it. Enjoy!