What pages have flashbacks that develop the understanding of Jay Gatsby?

Gatsby indulges in the use of flashbacks to keep alive his fantasy that someday he and Daisy will be able to shed other connections and complications and build a future together. ... He expresses his regret that Daisy didn't enjoy the party, then begins remembering earlier and better times in their relationship.

mark twain, real name samuel langhorne clemens, was an american writer, humorist, entrepreneur, publisher, and lecturer. among his novels are the adventures of tom sawyer and its sequel, the adventures of huckleberry finn, the latter often called "the great american novel". wikipedia