When you lift or do hours of cardio and your body says "UM I've had enough" the next day. You know the feeling, muscles that are so sore lifting a pencil hurts.....I lift the following day.
I'm not talking about lifting 120 lbs, just 10 lbs for some bicep curls, tricep curls, etc. Do a set or two, not to fatigue your muscles as you normally would with weight lifting, but to just stretch those muscles out. I feel better and recovery time is less so I'm ready the next day to continue.