I have to admit it's hard to think of a "good" day as something other than "doing lots of things". Having depression means I've had to adjust that way of thinking, & while I do love having a productive day, these days it's a measure of how depression is impacting me, more than my worth as a human being.
"Productive" is a good sign because it means my energy levels & mood are doing really well.
On other days, a "good" day means I was able to avoid crashing, or I ate enough to keep my mood going, or I took a shower.