Call the return location to confirm they do offer after hours return. If they do it usually involves a key drop box.

Write down the return date/time/fuel/plate number and drop it into the box along with the key. Putting the plate down on the details is important in case the return jacket is separated from the key in the box.

Return location would likely have security cameras as well so if you are scheduled to return on Saturday but actually show up on Sunday, putting down a Saturday return date/time would be rejected by the location if they decide to view their recordings.