A 24 hour rental period that extends beyond 11:59 PM Monday will count as a weekday.

I'm running into the same problem as CrazyOne for a January rental, picking up Sat AM and returning Tues PM. Sat-to-Sun and Sun-to-Mon are weekend days, and Mon-Tues and Tues AM-Tues PM are weekdays. But if I could switch my pickup to Friday evening, the 24 hour rental clock runs out Monday PM, and I now have 3 weekend days.

Since I'll have the time and be near the airport, my solution is to return my car Sat evening and start a new 3-day rental. That will get me 3 weekend days and 1 weekday. This saves me about $30, and for the Sat rental I'm going fullsize to get the ZE1 300 point bonus.