I've tried Fridays, Saturdays, Sundays and even Mondays. I've found that if I want to have a productive weekend I have to have my reviews on Friday afternoons or night. I usually start processing my electronic inboxes while I'm at work and my paper ones when I get home. People at work usually take off early on Fridays afternoon so I find it to be the perfect time to start review.
This way when I get up Saturday mornings I'm ready to hit the ground running. If I do my review on Saturday mornings I feel it slows me down. Having said that, it's still better than putting it off for later during the weekend because by the time the weekend rolls around I need a review to put things back in order to get anything done. And as for Mondays, forget it, too hectic at work.