Why don't you call them? They can do the award booking over the phone. It really depends upon exactly when you will be returning on Monday to determine if it's a weekend or weekday. It also varies by location.
Click "Redeem Points & Blackout Dates" for the peak period list. This link might work.
Keep in mind that Hertz has 50% off airport rental rates on the weekends now at most locations, so I think it might not be worth taking a weekend award in some cases if you pick up by end of month.