Hannah Stern's entire journey through the past leads her to better understand her family, her heritage, and herself. But, to get a little more specific, there are several key events in The Devil's Arithmetic that stand out as truly affecting Hannah.
The first, obviously, is the actual moment where Hannah is transported to the past. On the night of Passover, she is taken from 1980s New York to a small village in 1942 Poland. Here, she takes on the identity of a girl named Chaya, which just so happens to be Hannah's Jewish name.
When Hannah and other Jewish guests are on their way to a wedding, they are stopped by a group of Nazi officers, and forced onto a train headed towards a concentration camp. Several people die during the trip, including a girl Hannah's age named Rachel, who had wanted to be her friend.
Hannah and the others arrive at a concentration camp, and are given the infamous number tattoos that prisoners of these camps had. After having her head shaven, Hannah realizes that she has lost some of her memory, specifically of her old life in New York.
Hannah meets a girl her age named Rivka, who helps her to stay alive during her stay in the camp.
A very young boy named Reuven is chosen to be executed, despite Hannah's attempt to save him.
When their attempt at escaping the concentration camp fails, a group of prisoner's, including Hannah/Chaya's Uncle Shmuel, are executed.
When a guard is about to take Rivka to the gas chambers, Hannah takes her place, sparing Rivka's life. Instead of dying, Hannah is transported back to the present.
Hannah realizes that Rivka, the girl she saved, is her Aunt Eva. Hannah's Jewish name is Chaya because she is named after the girl who saved her aunt's life.