I was going to rant a bit in a new thread, but I'll bump this oldie because some of that first info may still be useful. Either way, I must share my annoying experience with "When does the weekend end".
The real answer appears to be: it depends upon what day you are returning the car! Argh!
I'm trying to rent from PIT on Sunday the 18th morning (or even Saturday 17th evening) returning Tuesday the 20th evening. So 3 days worth. Now, Tuesday is obviously a weekday, but it seems that Monday should be a weekend day. Well, it is, as long as I return by Monday evening. If I price Sunday and Monday, any combo of 2 days returning anytime on Monday, it's 2 weekend days (or 3 or 4 starting all the way back from Friday if I want). But as soon as I put Tuesday in there, it's not 2 weekend days plus 1 midweek day. It's only 1 weekend day, plus 2 midweek days! ...? Because I want the car until Tuesday, the rate for Monday changes. That is lame. I know, I know, it's yield control, supply-demand, much like the airlines, etc. But it feels somehow more in your face this way. I tried different CDPs, etc. Same result.
Oh, here's a new twist: I finally did try a Saturday night pickup, and suddenly that allows me to have 2 weekend days and 1 midweek day. So as long as I can get back Tuesday by hour of the pickup on Sat night, that should finally get me to where I need to be. Sheesh. Maybe I just needed to type it out to make sure I checked all the combinations....
Still, my original point remains: it does change depending on your return date. If you pick up Sunday and return Monday, that's 2 weekend days no problem, even without the Saturday night keep. If you pick up Sunday and return Tuesday that's 1 weekend day and 2 midweek days. The rate difference is particularly dramatic right now: weekday costs about twice the weekend rate.
For the heck of it, I checked some Tuesday morning returns and didn't find one that didn't charge at least one midweek day. That wouldn't help me in this case anyway, as I need to return on Tuesday night.