why do the rosens not celebrate jewish new year?

Ellen is very excited about the Jewish New Year and everything seems to be back to normal. However, on the day of the Jewish New Year, Mama announces that Mr. and Mrs. Rosen have had to leave Copenhagen and Ellen will be staying with the Johansens. Papa explains that the Germans are starting to arrest the country’s Jews. To stay safe, Ellen will pretend to be Annemarie’s sister.