Saturday: Chest & Abs
Sunday: Back & Calves
Monday: Off
Tuesday: Shoulders & Hamstrings
Wednesday: Arms & Abs
Thursday: Quads & Calves.
Friday: Off

Chest, focus on incline first as I'm going for more fullness with upper chest

Back - Two exercises with 4 sets each, then 3 exercises with 3 sets each. Always vary the exercises each workout.

Shoulders, Hamstrings, Arms, and Quads - always vary the exercises, but stay within a certain amount of sets for each.

Nothing that I've read, just experimentation over time. I'm 51 years old, and need a little more recovery time. This schedule allows me to train the muscles sufficiently to respond, with good recovery time also. Also, my experience has been that Mondays (at least where I've worked out at) are always busiest. By not training on Mondays, I find I'm better able to get to the machines & weights that I want to use.