In to Kill a Mockingbird, Atticus comes home in the afternoon during the missionary society meeting. Why?

In Chapter 24, Scout is invited to participate in her Aunt Alexandra's missionary circle. Towards the end of the chapter, Scout mentions that she heard Atticus come in the front door and was surprised because Atticus was never home that early. Atticus excuses himself for interrupting the missionary circle and asks to speak with Alexandra and Calpurnia in the kitchen. Miss Maudie and Scout join the three in the kitchen where Atticus tells Calpurnia that he wants her to go with him to Helen Robinson's house. Aunt Alexandra asks Atticus what's the matter, and Atticus tells them that Tom is dead. He tells them that the prison guards shot him while he was running towards the fence during an exercise period. The guards claim that Tom began to climb the fence and did not stop when they fired warning shots and gave him directives to climb back down. Atticus tells them that Tom was shot seventeen times, and he wants Calpurnia to go with him to help tell Tom's wife, Helen.