Simple. You just get up and do it. To me it's the same as going to work on a bad day. If your are having a bad day you don't think twice about going to work do you? I'm not sure why a bad day should have any effect on exercise.

Somedays I just do it, others I skip it and work out the next day. If I am having a really bad day not much I can do but sleep.

Just me, but I used to go to they gym 4 times a week and beat myself up if I didn't. I know I need the endorphins for my depression, but now I walk or cycle outside. On a bad day I shower, drive somewhere pretty and walk. Sometimes I'll run for a bit if no one is looking. Or I'll did a big hole in the garden & fill it in(!) Outside looks and smells better than the gym,and is free so I don't mind going as much. All exercise is exercise....

I might skip an intense or challenging workout, but I would definitely replace it with something like a long walk, at least.