Such thing as overtraining? Or is it in your head?

I snap shot this from IG, one of my followers posted it.

You know I do take one day off a week and sometimes I'm like "eh I feel like working out!" But the following day, I'm like "man I feel great and ready to go!"

This makes me think though...a crossfit mantra is "No days Off!" 

what do you guys think or believe?