When exercising abdominal muscles, is it best to do all of the exercises on one training session or is it best to have a different series of exercises for certain days of the week?

Say for example, on a monday I was to do abdmoninals. Would it be better to do a few movements for upper abs, mid-section abs, and lower abs, and then on a wednesday still focus on all three, but do different exercises?

Or should it be setup so that on a monday, only do upper abs. Then a wednesday would be for mid section abs and a friday would be for lower abs. Then on the weekend do obliques.

You can't separate the rectus abdominus: it's one muscle. And you don't need that many exercises/workout, either: you're better off choosing a functional movement pattern, such as standing rotation or dynamic stabilization stuff, IMO. The abdominal muscles, especially the deep transverse abdominus, are meant to work together...you're better off working them that way.