To put it briefly, sometimes. In your case (6-day rental becoming a 5-day rental) it should still keep the weekly rate so I don't think you'll have an issue; you may want to ask at the desk before driving off, though.

The situation where this would likely be an issue is if you have, for instance, a 5-day rental but return it after 4-days, meaning it no longer qualifies for a weekly rate; in that case they would have to change your rate to a daily rate (which can vary by weekdays and weekends) which can often cost more.

FWIW, unless you use the Amex Plat CDP, a full day is charged after two hours from the day-to-day (24 hour period) rollover on your reservation. So if your reservation begins at 10am on Monday, it will be considered a weekly (5+ days) reservation if your drop-off is at noon or later on Friday.

-J.