Why did you name the book NIGHT?
This book is called "Night" because we associate night with darkness and we associate darkness with evil. The book is a story about a time when evil took over much of Europe. Therefore, the title makes sense.
The Holocaust (which is, of course, what the book is about) was in my opinion the most clearly evil thing that has been done in the life time of anyone living today. This means that is appropriate to talk about it as a night, a time of darkness.
In my opinion, the book is also called "Night" because of the darkness experienced within the souls of those who went through it. The book explores, among other things, how the characters think about God and how they stay (or do not stay) sane through their ordeals. You could see these struggles as "nights," times of darkness for the victim's own souls and sanity.