Scout, Jem, and Dill are fascinated by the mystery surrounding Arthur (Boo) Radley. In Chapter 5, Jem and Dill plan on using a fishing pole to drop a note on Arthur's window sill. The note is intended to get Arthur to come outside. As Dill says, "We’re askin‘ him real politely to come out sometimes, and tell us what he does in there—we said we wouldn’t hurt him and we’d buy him an ice cream.” Dill thinks that Arthur might feel better if he came out of the house. However, Atticus scolds them for making a spectacle, something Arthur would certainly not want. Scout determines that if they really wanted Arthur to come out, the front door would be a much more appropriate way to go about it.
In Chapter 6, still curious, Jem and Dill decide to sneak a peak in Arthur's window. Since this was Dill's last night in Maycomb, this would be their last chance to steal a look at Arthur. Other than simply being curious kids, and this being Dill's last night, they give Scout strategic reasons for why they chose this night:
Because nobody could see them at night, because Atticus would be so deep in a book he wouldn’t hear the Kingdom coming, because if Boo Radley killed them they’d miss school instead of vacation, and because it was easier to see inside a dark house in the dark than in the daytime, did I understand?