period one year ago. Operating income for the second quarter increased 48% to $736 million (or 23.8% of sales) from $498 million (or 19.2% of sales) for the same period one year ago.
Net income for the second quarter ended June 30, 2020, increased $178 million, or 50%, to $532 million (or 17.2% of sales) from $354 million (or 13.7% of sales) for the same period one year ago. Diluted earnings per common share for the second quarter increased 57% to $7.10 on 75 million shares versus $4.51 on 78 million shares for the same period one year ago.
Year-to-Date Financial Results
Sales for the first six months of 2020 increased $568 million, or 11%, to $5.57 billion from $5.00 billion for the same period one year ago. Gross profit for the first six months of 2020 increased 11% to $2.93 billion (or 52.7% of sales) from $2.65 billion (or 52.9% of sales) for the same period one year ago. SG&A for the first six months of 2020 increased 4% to $1.77 billion (or 31.8% of sales) from $1.70 billion (or 34.1% of sales) for the same period one year ago. Operating income for the first six months of 2020 increased 23% to $1.16 billion (or 20.8% of sales) from $943 million (or 18.9% of sales) for the same period one year ago.
Net income for the first six months of 2020 increased $157 million, or 23%, to $832 million (or 14.9% of sales) from $675 million (or 13.5% of sales) for the same period one year ago. Diluted earnings per common share for the first six months of 2020 increased 29% to $11.06 on 75 million shares versus $8.56 on 79 million shares for the same period one year ago.
2nd Quarter Comparable Store Sales Results
Comparable store sales are calculated based on the change in sales for U.S. stores open at least one year and exclude sales of specialty machinery, sales to independent parts stores and sales to Team Members, as well as sales from Leap Day in the six months ended June 30, 2020. Online sales, resulting from ship-to-home orders and pick-up-in-store orders, for U.S. stores open at least one year, are included in the comparable store sales calculation. Comparable store sales increased 16.2% for the second quarter ended June 30, 2020, on top of 3.4% for the same period one year ago. Comparable store sales increased 7.5% for the six months ended June 30, 2020, on top of 3.3% for the same period one year ago.
Share Repurchase Program
In order to conserve liquidity in response to COVID-19, the Company suspended its share repurchase program on March 16, 2020. The Company continued to evaluate business conditions and its liquidity and, as a result of this evaluation, resumed its share repurchase program on May 29, 2020. During the second quarter ended June 30, 2020, the Company repurchased 0.2 million shares of its common stock, at an average price per share of $417.79, for a total investment of $77 million. During the first six months ended of 2020, the Company repurchased 1.7 million shares of its common stock, at an average price per share of $390.14, for a total investment of $651 million. Subsequent to the end of the second quarter and through the date of this release, the Company repurchased an additional 0.1 million shares of its common stock, at an average price per share of $423.09, for a total investment of $36 million. The Company has repurchased a total of 77.9 million shares of its common stock under its share repurchase program since the inception of the program in January of 2011 and through the date of this release, at an average price of $165.10, for a total aggregate investment of $12.87 billion. As of the date of this release, the Company had approximately $882 million remaining under its current share repurchase authorization.
Continued Suspension of 2020 Guidance
As previously announced, the Company withdrew all previously issued 2020 guidance, and given the ongoing uncertainty related to COVID-19, the Company is not resuming 2020 guidance at this time.
Non-GAAP Information
This release contains certain financial information not derived in accordance with United States generally accepted accounting principles (“GAAP”). These items include adjusted debt to earnings before interest, taxes, depreciation, amortization, share-based compensation and rent (“EBITDAR”) and free cash flow. The Company does not, nor does it suggest investors should, consider such non-GAAP financial measures in isolation from, or as a substitute for, GAAP financial information. The Company believes that the presentation of adjusted debt to EBITDAR and free cash flow provide meaningful supplemental information to both management and investors that is indicative of the Company’s core operations.