You can choose to upgrade as many or as few segments as you like. Use aa.com or the automated upgrade assistant (telephone) and choose the segments you want upgraded.
Most people prefer to save their upgrades for longer flights and sit in coach on shorter flights, but there's no reason you can't do it backwards. That said, F is usually more "impressive" on longer routes, with meal service and such.
Upgrade chances on saturday are probably good. Mondays are more difficult, especially mornings.