Generally, night is a time of day where everything is dark and mysterious, making things eerie and inevitable. The author may have used the word night as a motif because that was how each day was lived- no sunshine, only darkness. Another idea is from the Bible, specifically from Genesis 1:4-1:5.
"And God saw that the light was good. And God separated the light from the darkness. 5. God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day."
When I read the book over and over again, I thought perhaps it was this particular quote that may had inspired the book's title:
"So much has happened within such a few hours that I had lost all sense of time. When had we left our houses? And the ghetto? And the train? Was it only a week? One night- one single night" (34)?