Correspondence, letterbooks, writings, family papers, printed matter, photographs, and other papers relating to Day's life and his work as a pictorialist photographer and co-founder of the Copeland and Day publishing company, Boston, Mass. Documents his participation in the American Arts and Crafts movement in the 1890s, his philanthropic activities and relationships with a group of urban youth he met through his efforts with settlement houses in Boston, his role as a mentor to Kahlil Gibran, his chalet on the coast of Maine, and his varied interests including the poet John Keats, books, local history and genealogy, and horticulture. Other subjects include his connection to the Visionists, a group of artists and intellectuals in Boston, Mass.; the literary magazine, The Mahogany Tree; the social workers Jessie Fremont Beale and Florence E. Peirce; and the promotion of photography as a fine art by the pictorialist and Photo-Secession movements. Correspondents include Alvin Langdon Coburn, Herbert Copeland, Ralph Adams Cram, Louise Imogen Guiney, Bertram Grosvenor Goodhue, Gertrude Käsebier, Kihachirō Matsuki, Edward Steichen, Alfred Stieglitz, Clarence H. White, and Jane Felix White, and Day's parents, Anna Smith Day and Lewis Day.

Family papers include correspondence between Day and his parents, Anna Smith Day and Lewis Day, diaries, travel journals, school papers, photographs, and other papers. Topics include Day's early trips to Denver, Colo., and Europe; and his years at Chauncy Hall School, Boston, Mass.