The Day Development Company was created in October of 1930 when the Hercules Exploration Company, which had existed since November 1928, officially changed its name. Day Development was a geological exploration company whose purpose was to "seek for and secure openings for employment of capital in the U.S. and throughout the world, and the view thereto to prospect, inquire, examine, explore and test, to dispatch and employ expeditions, commissions, experts and other agents." The company was organized under the laws of Delaware and maintained an office there as well as in Wallace.

The stockholders in the company were Eleanor Day Boyce, Harry L. Day, Henry L. Day, Jerome J. Day, Wray Farmin, W.B. Heitfeld, F. Wallace Rothrock, Frank M. Rothrock, and William H. Simons. After Jerome's death in 1941, part of his stock went to his wife directly and part went into trusts for his wife and his daughter. Ownership of stock in the company was quite expensive. There were only 10,000 shares, each with a par value $100 and only 518 shares were ever issued.

Harry Day and Eleanor Day Boyce were the principal shareholders, each with over 100 shares. Apparently ownership of stock in the company was by invitation only. All the stockholders were either members of the Day family or close family friends. Whenever the company needed money, it invited the stockholders to buy more stock.

Information on how successful the mines were is not contained in this record group. Most of the correspondence represents the efforts of persons attempting to get the Days to invest in their properties. The exploration included properties primarily in the western United States. Several companies located outside the U.S., usually in the Northwest Territories of Canada, did ask Day Development to have a look at their property. But, for the most part Henry Day refused to expand out of the United States, no matter how good the prospect might have looked.

The board of directors of the Hercules Exploration Company consisted of Jerome, Harry, and Henry Day, Frank Rothrock, and J.M. Perry of Yakima Washington. John Wourms acted as designated agent until his death in 1945; Paul Jessup was appointed to replace him.

In 1936 Day Development sold all of its machinery. The company was dissolved on December 29, 1947 apparently because Day Mines had its own exploration arm.