At the end of Chapter 5, Atticus catches the three children making fun of Boo Radley. Atticus tells them to stop tormenting Arthur/Boo Radley. After this, they want to be stealthy in their games with Boo in order to avoid getting into trouble.
In Chapter 6, one of the many reasons Jem gives as to why they have waited until night time, is that Atticus will be so focused on reading that he won't hear them nor will he be concerned with what they are doing.
Being dramatic to the point of absurdity, but with some real common sense logic, Jem adds that by waiting until the end of summer, if Boo kills them, they will miss school instead of vacation.
Jem concludes with the practical reason that it is easier to see into a dark house when it is dark outside than it is when it is light outside.
Jem gives Scout strategic reasons as to why they have waited.
Perhaps there is another reason that they decide to get a glimpse of Boo at the last minute. Jem doesn't mention this overtly but since Dill is leaving, he might have a sense of urgency to do something daring with the little time they have left before Dill goes back home and before he and Scout return to school.