The coupon has very few particulars re the requirements. For instance, it doesn't say that a Sat. night overkeep is necessary, but the system won't accept it otherwise. So I made the reservation as returning early Sunday AM; either way it's a 2 day rental the way it's charged out. Will I encounter a problem (additonal charge due to coupon ineligibility) if I return the car "early" on Saturday evening. I'm not that familiar with National's practices, as I rarely use them.