What were the booms and lights on kirsti birthday?

On Kirsti's birthday in "Number the Stars" by Lois Lowry, the booms and lights refer to the celebration of the Danish Resistance's efforts against the Nazi occupation during World War II. The fireworks symbolize hope and freedom, contrasting with the oppressive atmosphere of the time. Kirsti, unaware of the deeper significance, simply enjoys the festive display, highlighting the innocence of childhood amidst a grim reality. This moment serves to emphasize the themes of resilience and the longing for liberation.