Projects are never done. You'll always be improving, releasing new versions, fixing security issues, making your application/product fit in with the rest et cetera.
A good moment to show HN is, I guess, the moment where your potential users can really see what your app will be like when it's near completion, in a way that they will share it with others. And about your "product": good luck!
That quote is a derivative of a long known anonymous quote, "It's easier to be finished, than to be satisfied". You can call it done whenever you want - or tweak it forever . . .
Any time will do. Someone analyzed best times and posted an article here a while ago, but I've seen show hn's posted on Sunday afternoons stay on the front page till Monday. Just ship it.