It's funny you chose that as your example, because there are very strict definitions of when day becomes night. I think what you were looking for was "when does someone become bald" or "when does an acorn become a tree".

There is a reason those are classic philosophical questions. Because they highlight the fact that while it is easy to identify the ends of the spectrum, it's impossible to find the midpoint, because everyone has a different lived experience.

>there are very strict definitions of when day becomes night

Hmm ... that has not been my experience.

>it's impossible to find the midpoint, because everyone has a different lived experience

Yes! That's the answer to your question.