In my experience 'bad' days are caused by having low energy, for instance on an evening after a long day of school or work. Even if I warm up thoroughly, I never get that comfortable feeling on those evenings. Even a few big mugs of coffee or energy drink won't help. In my opinion, if you are still in the mood for piano even if you're tired, keep those sessions to stuff like theory / learning new music / scales / slow play / or they might get counterproductive. Or just skip the session, and try again the next day when you have more energy. Besides having low energy, there will also be random days when you just suck, like is the case in every other instrument or sport.