The papers contain letters, writings, and records of the Day family. Series I holds letters sent to Horace Day from his parents Gad and Roxanna Day, his brother George Edward Day (Yale 1833), his cousin Lucinda Day, and his friends, Yale classmates, and colleagues including Edward E. Atwater, Erasmus Darwin Hudson, Charles Backus McLean, and Noah Porter. Also present are letters of recommendation written on Day's behalf by Jeremiah Day, Eleazar T. Fitch, Anthony D. Stanley, and Nathaniel W. Taylor. Personal Papers includes material investigating Gad Day's military service and a proposal from E. S. Goodrich to publish the Pioneer newspaper at St. Paul, Minnesota. Also present is a manuscript broadside announcing a music concert at sea and a printed broadside advertising Thomas Kimpton's Parcel Express shipping service from London to "all parts of the kingdom" as well as the colonies and and North America; they may be related to Day's trips to Europe in the late 1850s. Series II holds autograph manuscripts and typescripts of writings by Day and his daughter Sarah Day Woodward, including addresses delivered before the New Haven Colony Historical Society. Also in the series are a group of manuscript sermons by an unidentified author, dated 1728-1833, and newspaper clippings related to the history of New Haven, Connecticut, dated 1857-1915. Series III holds five financial ledgers for the Day & Fitch company dating 1845-1855, and one personal account book recording Day's expenses, 1868-1874. Series IV holds two volumes of Day's notes made during meetings of the New Haven Board of Education, 1882-1890.