Elie Wiesel's novel Night depicts his first hand experiences during his imprisonment at Auschwitz during the Holocaust. While one could justify the title of the novel in different ways, the title has been known to refer to two very specific ideas.
First, readers of the novel are very familiar with Elie's loss of faith in God. During his time at Auschwitz, Elie's faith hits an "all time low." That being said, some could support that his loss in faith placed him in darkness. Given that light did not exist until God created it, his loss of faith puts him into a place where his is not able to find God or his light. Therefore, he exists in darkness--or night.
Second, Elie cannot come to understand why the violence and brutalization in the camps exists. Given that he is not "enlightened" on these happenings, he is in the dark about why the camps even exist.
Although you only needed two reasons, one last reason needs to be mentioned. The original title of Night was And The World (Has) Remained Silent. Given that the "world" did nothing to stop the Holocaust, at first, Elie could have felt that the world turned its back on those victimized, brutalized, and killed during the Holocaust. The world essentially closed its eyes to the truth. Therefore, the world was dark--like night.
Elie Wiesel is said to have named his novel "Night" due to the book being written about the darkest period of his life. The book centers around the hardships he experienced during his time spent in a Nazi concentration camp.