In "Night", why is there joy when the signal finally comes for them to leave Sighet and the ghettos?
Elie makes a point to tell the reader how the Jews of Sighet lived until 1942 thinking that the war would never reach them. They saw the war as something abstract and they refused to believe the atrocious stories they heard about what the war had done to its neighbors. The Jews of Sighet didn't want to believe that evil at that level existed. When Moishe the Beadel escaped to return and save the Jews of Sighet by telling his story they ignored him. When the soldiers finally showed up one day they acted much like Germany did under the Nazi regime, they were kind, they "weren't that bad", they were well mannered, and once they had gained the trust from the Jews of Sighet they were already virtually powerless to stop it.
Since the Jews of Sighet had not heeded the warnings of others when the suffering began they believed that it would only be for a short while and then things would get better, they would be moved to be kept safe from the war, the soldiers wouldn't hurt them all. When they were standing on the streets in the hot sun to be moved out of the ghettos and "away from the war" to the camps where so many died, they thought that standing in the sun without water was the worst torture they would have to endure. When they finally left they were just relieved to get out of the sun and onto a train and again they thought, wrongly, that they would be safe from any further torture.