There are many important events in this book, which interweaves many family stories. I like to start with the first major event, which is the Corderos moving into the house on Mango Street. It is a major event for them (their first house), and is therefore full of expectation and emotion.
Another event that defines the book is when Rachel and Lucy new little sister dies. This loss is sad in itself, of course, but it is also when Esperanza gets her palm read. This sets up her wish in the final chapter, and the expectation that it might happen: that she might have her own house some day.