It really depends. You could do some cardio-based stuff on Saturdays like interval training for endurance purposes and so on, and that would be completely fine, as long as it's not heavy lifting aimed towards strength gains, because your muscles and nervous system do need that 48 hour period to adapt to the stressors you put on them during your heavy lifting sessions, so that you can continue to progress.

So if you want to do some plyometrics and light dumbbell work and interval training and accessory lifting and so forth, have at it. That's not gonna hurt your barbell lifting any, especially if you're young and your body bounces back much faster and adapts much quicker than with us older folks.