When considering five major events in Harper Lee's To Kill a Mockingbird, one might focus on those that contributed the most to a major theme of the story, such as the loss of social innocence in Scout and Jem, as well as the fact that the book is a bildungsroman—a coming of age story. I would ask myself what events contributed most to the children learning about the world's messes and how there are no easy answers to why people act the way they do.
First of all, one of the biggest mysteries for the children is Boo Radley. Is he a monster or not? During the first summer, the children act out stories about Boo and even spy on him. After the episode involving the house fire, Jem realizes the following:
"but Atticus, I swear to God he ain't ever harmed us, he ain't ever hurt us, he coulda cut my throat from ear to ear that night but he tried to mend my pants instead ... he ain't ever hurt us, Atticus—"
[ ... ]
"Thank who?" I asked.
"Boo Radley. You were so busy looking at the fire you didn't know it when he put the blanket around you." (72)
Next, there is a big change for Scout when Aunt Alexandra comes to live with the family in chapter 13. She comes because Atticus knows the upcoming trial will take a lot out of his time and energy, but also probably because he knows that the children need a female family member who can teach them the ways of society—especially Scout, who "needs" to learn how to be a lady. She doesn't like it, but Scout learns about how white women behave through her aunt's example and through the social gatherings of the missionary circle at her house.
A third event that sheds light on Scout's maturing over the course of the novel. Scout goes from shouting out loud that Walter is crazy for pouring syrup on his food to pondering the inconsistencies between what adults say and do. For example, Scout nails Ms. Gates's hypocrisy when she tells Jem the following:
"Well, coming out of the courthouse that night Miss Gates . . . was talking with Miss Stephanie Crawford. I heard her say it's time somebody taught 'em a lesson, they were gettin' way above themselves, an' the next thing they think they can do is marry us. Jem, how can you hate Hitler so bad an' then turn around and be ugly about folks right at home—" (247)
Another major event is, of course, the trial of Tom Robinson. Not only does this gather the whole community together under one roof to discuss complicated and intense issues, but it is a definite learning experience for the children. They see their father over the course of the year as he prepares for and then presents a strong case for a black man in front of a white jury. Then, when the case is lost, the children must deal with difficult feelings from which they must learn some hard lessons.
Finally, when Mr. Ewell attacks the children on Halloween, not only is Boo Radley permitted to prove his good nature by protecting the children, but Mr. Ewell meets karma at the end of his own knife. As justified as that might be, Jem doesn't escape unharmed, because his arm is mangled to the point that he never gets to play football competitively afterwards, which was a dream of his. That's a pretty major event in Jem's life, but Scout gets to meet her hero and walk him home that night, which solidifies the fact that just because someone thinks another person is a monster simply because they are different does not mean that they should be discriminated against. In fact, those people who seem different to one might wind up being a hero to many others.