September 15
Individuals: Pay the third installment of 2026 estimated tax if your withholding is not expected to cover enough of your tax.
Calendar-year corporations: Pay the third installment of 2026 estimated income tax. Corporate deposits generally must be made electronically.
S corporations: File the 2025 Form 1120-S if the business received a six-month extension, furnish Schedule K-1 to each shareholder, and pay any tax, interest, or penalties due. Businesses with certain foreign activity may also have Schedule K-2 and K-3 requirements. Certain 2025 employer retirement-plan contributions may also be due.
Partnerships: File the 2025 Form 1065 if the partnership received a six-month extension and furnish Schedule K-1 to each partner.
Employers and withholding agents: Make August monthly deposits of payroll and nonpayroll federal taxes if the monthly deposit schedule applies.
Trusts and estates: Pay the third installment of 2026 estimated income tax, if required.
September 30
Calendar-year trusts and estates: File the 2025 Form 1041 if a five-and-a-half-month extension was filed, and pay any remaining tax, interest, and penalties.
October 13
Employees who received $20 or more in tips during September: Report those tips to the employer. The usual October 10 deadline moves to October 13 because October 10 falls on a Saturday and October 12 is a federal holiday.