How did Elie Wiesel use night as a symbol of what was happening to the Jewish people?
There are several ways that Elie Wiesel used the concept of "the night" in his book. First, night has often been seen as a symbol of death in literature. One thinks of Dylan Thomas's "Do Not Go Gentle into that Good Night." Thus, night can be a symbol of death, an appropriate symbol for a book that featured multiple (and often graphic) deaths.
For children, night can also be seen as a time when shadows appear and when monsters are lurking. Again, the symbolism is appropriate with regard to what is happening in the book. Elie, a child himself, is facing the monsters of Dr. Mengele, the SS officers, etc. Thus, like a long and fearful night for a child, Elie is left to deal with monsters all his own.
Finally, the night can seem a long stretch, particularly for those unable to sleep. The concept of night as a long stretch is seen directly in Night, as when Wiesel writes in Chapter 3, "Never shall I forget that night, the first night in camp, which has turned my life into one long night ..." Thus, the night comes to symbolize several aspects of the story.