Whoever said you need a days rest between core training is dead wrong. The tissue is the same kind as that which makes up your jaw and calves. It is neigh on impossible to overtrain your core.
The abdominal muscles are just like any other muscle in the body. They muscle fibers tear and repair themselves. If you are working them everyday, when do they have time to repair? If you want to train properly, I would hit them 3-4 times a week. Oh.. and ANY muscle can be overtrained.. including your core.