This depends on which bombing you are referring to--this was during World War II after all, where bombing occurred all across Europe. However, the main bombing that affected the characters in this story comes in at the very end of the tale, and occurs in the city where most of the main characters are living, in Molching, Germany. Bomb raids had been occurring in the city for quite some time, enough for the citizens to have organized themselves into groups with specific locations to go in case there was a bombing in their area. And, they practiced those bomb drills several times, huddling in the basement of a neighbor's house, terrified, waiting for bombs to come, that never did at those times. Unfortunately, the bomb that wipes out most of Liesel's neighborhood occurs while most people are sleeping, in the middle of the night. The bomb wipes out buildings, and all of the characters that Liesel knows and loves--Rudy, her foster parents, and her neighbors. It is a tragic ending, one that Liesel escaped merely through chance; she was up late, down in the basement, working on writing her memoir. That is the only reason that she escaped the same fate as her foster parents and best friend Rudy.
Many cities in Germany were bombed during World War II, wiping out thousands of German citizens, whether they were complicit in the war crimes or not. Molching just happened to be one of those cities, and Liesel's neighborhood was an unfortunate target that the bombs did hit. That bomb brings the tale to a pretty climactic ending for the most part, only wrapping up entirely a bit later as some loose ties are closed off. I hope that those thoughts helped; good luck!