On a good day of writing I make discernible progress on a specific chapter, and the light-bulb is flashing with good ideas every 15 minutes or so. There are some sentences I am happy with.

On a normal day, words are added to the word-count. Some progress is made toward completion of something. The passive voice is abused.

On a below average day, I have the satisfaction of keeping the project going, even without making very great progress.

So there are really no bad days.