Clarence Shepard Day, Jr. (November 18, 1874–December 28, 1935) was an American author. Born in New York City, he graduated from St. Paul's School and Yale University in 1896. The following year, he joined the New York Stock Exchange, and became a partner in his father's Wall Street brokerage firm. Day enlisted in the Navy in 1898, but developed crippling arthritis and spent the remainder of his life as a semi-invalid.