I worked at Hertz for many years and I would imagine all the companies work pretty much the same.

In order to get the weekend (discounted) rate, you have to pick up by a certain time (I think for us it was 3 PM). Otherwise, all the business travelers that fly in Sunday night for a Monday meeting would want it.

The same is true on Thursdays when the weekend rates start. Our started at noon, and if you picked up at 11:59 AM, you would be charged the regular weekday rate. We always had a line up of people who intentionally booked at noon to get a cheaper deal, but showed up at 8 AM or 10 AM but had to wait. Part of the issue on Thursdays was fleet availability, as cars didn't start returning until late morning.