It's on pg. 134 at the bottom. It's used like this: "Makes no difference if it's Saturday," she said obscurely. (I have the version in a greenish color with a black tree on it). Hope this helps!
In chapter 10 Atticus and Miss Maudie mention that "It is a sin to kill a mockingbird." It plays into the title of the book, and also the theme of the book.