What were the booms and light in the sky on kirsti birthday?

On Kirsti's birthday in "Number the Stars" by Lois Lowry, the booms and lights in the sky were caused by the German military's bombardment of Copenhagen. The sounds of explosions and flashes of light reflected the tension and danger of the Nazi occupation during World War II. This event serves as a stark reminder of the harsh reality surrounding the characters, contrasting with the personal celebration of Kirsti's birthday.