Eugene Rufus Day, the second of the sons of Henry & Ellen Day, was born in 1873. He studied at Santa Clara College in 1891, and showed an early interest in animal husbandry. He took an active role in the mining business of the family, and took part in the development of the Hercules Mine, serving as manager from 1912 until his death in 1922. In addition, he was associated with the ranch in Osburn, Idaho, the Day dairy (Standard Dairy Company), and the general store in Wardner. In 1909 he bought half interest in the Coeur d'Alene Hardware and Foundry, of which he was president from 1913-1922. A subsidiary of this company, beginning in 1915, was the Wallace Powder Company. Eugene was also the proprietor of Burke Teaming, a company owned by Henry L. Day and Son, meaning "Pa" Day and Eugene.

Plagued by illness all his life, he was partially paralyzed from birth. He also suffered from gallstones and Bright's disease which was the official cause of his death.

The major tragedy in Eugene's life was his disastrous marriage to Agnes Loretta Quinlan, daughter of a Mullan miner, on January 25, 1905. They separated soon after the marriage, but were never officially divorced. Eugene left nothing to her in his will and her feud with the Day family over the actual worth of the estate prolonged the probating of the will for several years.

Agnes Day's demand to know the net worth of the Day family in 1922 was similar to a demand previously made by the courts in 1917-1918 during what came to be known as the Cardoner Case which concerned the Hercules Mine. This case pertained to Eugene because Eugene was the principal in the case brought by the widow of Damian Cardoner, a former partner in the Hercules. In 1917 Mrs. Cardoner had accepted an offer from the Days for her share of the mine. Unfortunately for Cardoner that year was the most profitable in years for the Hercules. When she discovered this she claimed the Days had deceived her about the mine's true worth. The courts exonerated the Days, saying they gave a fair price for the widow's share.

These cases allowed the public to discover the exact worth of the family's empire; which because it had been such a closely guarded secret before this time, was a matter of much curiosity. The Days had a well-known penchant for financial reticence, apparently dating from the fights with the smelter trusts at the beginning of the century.

Eugene was principal promoter and president of two unprofitable mining companies, Ray-Jefferson, 1915, and Amazon-Manhattan, 1917-1922. Despite these failures his reorganization of the Coeur d'Alene Hardware & Foundry and Sherman Lead proved sound and profitable.

Near the end of his life he allied himself with Sarah Smith, widow of a discoverer of the Hecla Mine in an unsuccessful suit to control that company. On February 11, 1922, eight days after the Hecla suit ruling Eugene Day died in Sacred Heart Hospital, Spokane.