Why does Jem want to go out at ten o'clock at night in To Kill a Mockingbird?

Jem wants to go downtown at ten o'clock at night because he is worried about his father, who has taken a lightbulb and an extension cord and then driven off to town.

Earlier on this Sunday, men from town have visited Atticus, asking if the Tom Robinson trial could not have a change of venue. Mr. Link Deas is heard saying to Atticus, "You got everything to lose from this, Atticus. I mean everything." Overhearing the men talking, Jem grows concerned, and as the phone rings, he calls out to his father. This breaks the tension; the men laugh and disperse.

That evening, however, Atticus gathers a light bulb and an extension cord, bidding everyone good-night,

"You folks'll be in bed when I come back, so I'll say good night now."

Because of the events of the day and this irregular behavior of Atticus, Jem begins to worry about his father. At 10:00 p.m., when Atticus has not returned, Jem tells his sister that he is going downtown, but Scout insists that she accompany him. As they cut through Miss Rachel's side yard, they tap on Dill's window, and in a few minutes, he joins them. Jem tells Dill he has a feeling that something is going on, so they head to Atticus's office, which is located in the Maycomb Bank Building. Atticus is not there, so the children walk up the street and see Atticus's lightbulb over the jailhouse door.

He was sitting in one of his office chairs, and he was reading, oblivious of the nightbugs dancing over his head.

Then, four dusty automobiles pull up and men with hats pulled low on their foreheads emerge from the cars to approach Atticus, speaking in whispers as they walk. When Atticus sees Jem, he tells him to go home, but Jem shakes his head. One burly man grabs Jem, pulling him nearly off his feet. This action angers Scout, who kicks the man in the shins, prompting Atticus to quickly scold Scout.

Looking around, Scout finally recognizes one of the sullen, tired men as Mr. Cunningham; she speaks to him and causes him enough discomfiture that he decides to leave, calling to the others to do the same. Afterwards, Mr. Underwood emerges and tells Atticus he has had him covered with his rifle all the time. The Finches all walk home, safe and content with one another.