A bad day writing isn't really a bad day. It's a setup for a good day.
Sure, you might spend all day beating your head against some portion of an article or some small section of your book, all the while feeling like you're making zero progress. But you are making progress. The text you're struggling with is slowly getting softened up for an upcoming good day.
And that good day might come tomorrow, so get ready for it: get to bed early, get a good night's sleep, and be sure to set out your work the night before so you can attack it first thing in the morning.
It may not feel like it, but you make progress on bad writing days too. And you're getting closer to a huge day--a truly good day with progress far beyond the ordinary.
Keep at it. It's coming. You'll see.