Make up a workout that you missed from the previous 3-day cycle or work on a skill/weakness.
“Now if you are going to win any battle you have to do one thing. You have to make the mind run the body. Never let the body tell the mind what to do. The body will always give up. It is always tired in the morning, noon, and night. But the body is never tired if the mind is not tired.”
–George S. Patton