When does Holden say he wants to be the catcher in the rye?

Holden Caulfield expresses his desire to be the "catcher in the rye" during a conversation with his sister, Phoebe, when he explains his fantasy of wanting to protect children from falling off a cliff while playing in a field of rye. He sees himself as a guardian who catches kids before they lose their innocence and fall into the adult world's complexities and corruption.