The papers of Harry L. Day are divided into four series.

The first series, General Correspondence and Related Records, had been subdivided into three subseries. The first of these contain letters relating to the Hercules Mining Company and the Day family, taken from Harry Day's letter cases, probably by Henry L.V. Day, for the use of C.R. Stark or John Fahey, or both, in their research projects concerning the Day properties. They are in approximate chronological order.

The letters in the second subseries were originally stored in letter cases labeled "Personal letters, bills, receipts." It is dominated by correspondence which related primarily to the mining businesses of Harry and other Day family members. Also included are items concerning politics and legislation as it affects mining, the Roman Catholic Church, and club memberships. The bills and receipts are chiefly for personal and household expenses. The subseries is arranged first by year and then alphabetically by personal or corporate names, using only the initial letter.

The third subseries contains similar material but is arranged alphabetically using both initial letter and full name designations. Included are correspondence, reports, claims, maps, contracts, settlements, and other records relating to the operation, production and proposed consolidation of the Hercules and Tamarack and Custer mining companies. There are also files on several smaller companies such as the Headlight, Hummingbird, Morning, and Western union. This subseries also contains personal correspondence and other records relating to Henry, Eleanor, Jerome, Blanche, and other Day family members. Of special interest is a letter from General W.T. Sherman dated February 9, 1848 and a 1882 photograph of the general autographed at Fort Boise.

Series two contains records concerning Harry's interest in several areas. The first of these is the State Bank of Commerce which contains copies of letters concerned with the grand jury inquiry into his service as bank director, 1909-1911, and his purchase of the bank's assets in 1913. Also included is a report on the investigation of the bank by Bland & Wilson, prepared in 1912. The correspondence in this subseries in alphabetical order and appears to have been refoldered by Henry L. Day in the 1970s. The records relating to his presidency of the Federal Mining and Smelting Company, 1912-1921, contain correspondence, financial statements, records relating to the Federal M & S vs. Washington Water Power Company suit, and transcripts of the testimony in the case of Star Mining Co. vs. Federal Mining & Smelting Company in 1921.

The Property and Investment records contain correspondence, certificates of incorporation stock certificates, balance sheets, location notices, abstracts of title, insurance policies, deeds, and other material related to the mining interests of Harry Day. The real estate records include tax sale certificates, tax receipts, land patents, indentures, bills of sale, etc. for properties in Wallace, Idaho; Santa Barbara, Dunsmuir, and Richmond, California; Portland, Oregon; and Spokane, Washington. The items in boxes 9-10 were originally in bundles and were placed in unlabeled folders by the original processor, therefore the folder headings are arbitrary, many taken from the inventory prepared by Day when he examined the material in 1972 (e.g. Bundle #1, Small cardboard box.) Material was arranged alphabetically by subject. Of interest in the Small Cardboard Box folder is a list of the property given to Henry by his parents when they retired to California in 1934. Also included in this series are reports of detectives working on the Eugene Day divorce case. The remaining material in this series was in labeled folders, two boxes relating to mining and one to bonds and other investments held by Day. These retain their original alphabetical order.

The third series contains financial material. Included are bills and receipts relating to the Day's estate Las Palmas in Santa Barbara, California, financial ledgers, voucher registers, and cash receipt books. Some of the ledgers were also used by Henry L. Day which accounts for the later ending dates for these items. The personal voucher registers for February 1928 to October 1930, and August 1935 to September 1937 are missing. The miscellaneous voucher register appears to relate to the family's interest in the Day businesses.

The other records in the final series are two date books which indicate where Harry was on each day and who he was with, and an ore shipment record for the years 1905-1906.