For me, I describe bad playing days as not being able to get a single song i play on point in terms of picking and fretting hand accuracy. that frustrates me beyond all hell. But i did start the practice session "Trying to play a song differently" than what i usually do. Longview by green day. After that, my fretting and picking hand would not stop messing up lol maybe just cuz i tried practicing something new before everything else. Ive noticed that happens after i try to learn a new song which is why i try to save learning new songs for the last half hour of practice. So what do you guys describe as a "Bad playing day" for you?