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 North America; they may be related to Day's trips to Europe in the late 1850s.