from new stores opened over the past twelve months. Net sales in the thirteen weeks ended September 26, 2020 were adversely impacted by decreases in retail store sales resulting from decreased traffic in our stores from customers staying at home in response to the COVID-19 crisis.
Gross profit. Gross profit increased $62.7 million, or 112.9%, to $118.2 million for the thirteen weeks ended September 25, 2021 from $55.5 million for the thirteen weeks ended September 26, 2020. As a percentage of net sales, gross profit was 37.8% and 30.1% for the thirteen weeks ended September 25, 2021 and September 26, 2020, respectively. Gross profit increased primarily due to higher sales. The increase in gross profit rate of 770 basis points was driven by 410 basis points of leverage in buying and occupancy costs as a result of expense leverage on higher sales, and a 360-basis point increase in merchandise margin rate. Merchandise margin rate increased 360 basis points primarily as a result of better full-price selling and growth in exclusive brand penetration.
Selling, general and administrative expenses. SG&A expenses increased $22.6 million, or 49.7%, to $68.0 million for the thirteen weeks ended September 25, 2021 from $45.4 million for the thirteen weeks ended September 26, 2020. The increase in SG&A expenses was primarily a result of higher store payroll, higher overhead and increased marketing expenses in the current-year period compared to the prior-year period which was impacted by COVID-19. As a percentage of net sales, SG&A decreased by 290 basis points to 21.8% for the thirteen weeks ended September 25, 2021 from 24.6% for the thirteen weeks ended September 26, 2020. SG&A expenses as a percentage of net sales decreased by 290 basis points primarily as a result of expense leverage on higher sales.
Income from operations. Income from operations increased $40.1 million, or 399.0%, to $50.1 million for the thirteen weeks ended September 25, 2021 from $10.0 million for the thirteen weeks ended September 26, 2020. The increase in income from operations was attributable to the factors noted above. As a percentage of net sales, income from operations was 16.0% and 5.4% for the thirteen weeks ended September 25, 2021 and September 26, 2020, respectively.
Interest expense. Interest expense was $1.2 million and $2.4 million for the thirteen weeks ended September 25, 2021 and September 26, 2020, respectively. The decrease in interest expense was primarily the result of a lower debt balance in the current-year period compared to the prior-year period.
Income tax expense. Income tax expense was $11.1 million for the thirteen weeks ended September 25, 2021, compared to $2.0 million for the thirteen weeks ended September 26, 2020. Our effective tax rate was 22.7% and 25.6% for the thirteen weeks ended September 25, 2021 and September 26, 2020, respectively. The tax rate for the thirteen weeks ended September 25, 2021 was lower than the tax rate for the thirteen weeks ended September 26, 2020, primarily due to a higher tax benefit due to income tax accounting for share-based compensation compared to a lower tax benefit in the thirteen weeks ended September 26, 2020.
Net income. Net income was $37.9 million for the thirteen weeks ended September 25, 2021 compared to $5.8 million for the thirteen weeks ended September 26, 2020. The increase in net income was primarily attributable to the factors noted above.
Adjusted EBITDA and Adjusted EBIT. Adjusted EBITDA increased $40.6 million, or 211.5%, to $59.8 million for the thirteen weeks ended September 25, 2021 from $19.2 million for the thirteen weeks ended September 26, 2020. Adjusted EBIT increased $40.2 million, or 310.8%, to $53.1 million for the thirteen weeks ended September 25, 2021 from $12.9 million for the thirteen weeks ended September 26, 2020. The increase in Adjusted EBITDA and Adjusted EBIT was primarily a result of the year-over-year increase in income from operations driven by an increase in net sales and gross profit and a decrease in SG&A as a percentage of net sales.
Twenty-Six Weeks Ended September 25, 2021 Compared to Twenty-Six Weeks Ended September 26, 2020
Note: Comparisons to the twenty-six weeks ended September 26, 2020 reflect the effect the COVID-19 crisis had on our results during such time period.
Net sales. Net sales increased $286.8 million, or 86.3%, to $619.0 million for the twenty-six weeks ended September 25, 2021 from $332.3 million for the twenty-six weeks ended September 26, 2020. Consolidated same store