In "To Kill a Mockingbird", why did Jem & Dill wait until the end of summer to spy on Boo Radley, and when they did spy on him what happened?
They probably waited so long simply because they had been slowly progressing in their bravery in regards to the Radley place throughout the summers. The first summer, they were brave enough to slap the house, and not much else. Later, when Atticus caught wind of their Radley obsession, he forbade them from doing anything more, and told them to leave the man alone. So, they had to do with just playing a sort of pretend game for a while.
At the end of the summer, Dill is about to leave, and their gathered courage, along with Dill's impending departure, prompt them into one last adventure. They sneak into the yard, peek into the window, and then see a ominous shadow coming their direction. Scared, they bail, and Jem loses his pants on the garden fence along the way. Mr. Nathan Radley even fires a shot from his shotgun at the intruders. Mr. Radley didn't know it was the kids, thankfully, and the kids get out of the fix by saying Jem lost his pants while they were playing strip poker with matchsticks.
That's the summary of what happened, but I also provided a link to a more detailed analysis below. I hope that helps!