I would say that the first main event that happens is Bud being moved from the Home to the Amos house. Bud's done this kind of thing before, and he knows what to expect out of process. In fact, he nails it right on the head with what to expect out of an older brother.
Main event 2: Bud having the pencil stuck up his nose and retaliating. That led to him being locked up in the shed, which led to him running away.
Main event 3: Bud meeting up with Bugs and the two of them going to Hooverville.
Main event 4. Deciding to search for Calloway and head to Grand Rapids
Main event 5. Bud meets Lefty Lewis who knows who Calloway is.
Main event 6. Bud finally meets Calloway and discovers that he is not Bud's father. He is Bud's grandfather.