What events happen in the middle of the story?

It would suffice to say that the middle part of Night is that section beginning with chapter 3 as Elie and his family arrive at Birkenau, the reception center for Auschwitz, and ending in chapter 5 with the evacuation of Buna. During this time, Elie and his father endure the barbaric conditions of two Nazi concentration camps. Elie loses his mother and little sister as they are separated at the infamous "selection" conducted by Dr. Mengele. Elie stays with his father, and the two are given advice to lie about their ages. They are marched past flaming pits where Nazi soldiers are throwing the bodies of children. It is here where Elie begins to doubt God, saying "Never shall I forget those moments which murdered my God and my soul and turned my dreams to dust." Amazingly, father and son survive and, after showers, hair cuts, and changing clothes, they arrive at Auschwitz where they spend three idle weeks.
During this time they experience the cruel reality of the camps where "gypsies" and "Kapos" control things. Elie's father is often beaten, especially when other prisoners find out Elie has a gold tooth, eventually blackmailing him out of it by threatening his father. Some Kapos seem benevolent such as the one who controlled the "musician's block" where Elie and his father live during their time at Buna. Their days are occupied with counting and sorting electrical equipment. Here, Elie meets a young French girl who offers him some bread after a beating by the tyrannical Idek. Elie meets the girl again many years later in Paris.
Elie and his father also endure the many selections. After the Jewish New Year, a new selection strikes fear in Elie as his father's health has deteriorated, and he fears the old man will be taken. The father even gives Elie his knife and spoon while saying goodbye. Miraculously, the father is spared.
Elie witnesses the many executions in the camp, such as the hanging of the young pipel who doesn't die immediately, because he doesn't weigh enough for his neck to instantly break but is left to strangle at the end of the rope. This sight causes Elie to further question God.
In chapter 5 Elie is stricken with a badly swollen foot and ends up in the infirmary where he undergoes surgery by a kindly Jewish doctor. While in the hospital, Elie hears rumors about the impending invasion by Russian forces. The Nazis make plans to evacuate Buna. Because Elie and his father are in the hospital, they are eligible to remain there. In a bitter irony, Elie and his father choose to leave, evacuating to Buchenwald, when staying would have meant earlier liberation by Russian troops.