The Devil's Arithmetic opens with what event, and why is it important to Hannah's family?

Passover Seder is important to Hannah’s family because it is about remembering people they lost during the Holocaust.

The novel opens with Passover Seder, and this is an important Jewish holiday.  Hannah does not want to go, because she and her brother will be the only kids there.  She does not understand the importance of the holiday, and its significance to her family.  Hannah thinks all holidays are alike.

"Passover isn't about eating, Hannah," her...

Passover Seder is important to Hannah’s family because it is about remembering people they lost during the Holocaust.

The novel opens with Passover Seder, and this is an important Jewish holiday.  Hannah does not want to go, because she and her brother will be the only kids there.  She does not understand the importance of the holiday, and its significance to her family.  Hannah thinks all holidays are alike.

"Passover isn't about eating, Hannah," her mother began at last, sighing and pushing her fingers up through her silver-streaked hair.

"You could have fooled me," Hannah muttered.

"It's about remembering." (Ch. 1)

The holiday is about remembering family members and friends that were lost during the Holocaust in World War II.  This was a significant event where almost every Jewish family lost someone.  The Nazis rounded up people in Germany and elsewhere in Europe and systematically exterminated as many as they could, killing millions.

Hannah has grown up hearing about the Holocaust, but she never really understood it.  The Holocaust had a huge impact on her family, but the meaning is lost on her.  She doesn't appreciate it because it is something she has always known but never understood.

You have to remember how much family means to them. Grandma lost both her parents to the Nazis before she and her brother managed to escape. And Grandpa . . ."

"I remember. I remember. . . ," Hannah whispered.

". . . Will lost everyone but your Aunt Eva. A family of eight all but wiped out." (Ch. 1)

Hannah is about to get an understanding of what actually happened and what the Holocaust was like.  She goes down a sort of magical rabbit hole into World War II.  She learns more about her namesake, Aunt Eva’s friend, who died in the camps.

Sometimes it is difficult for children to appreciate the significance of holidays.  One holiday can run into the next.  Passover Seder is important for Hannah’s family because it is an important Jewish holiday, but also because it is about remembering who they lost when they lost so much.