September 16
Individuals: Pay the third installment of 2024 estimated taxes (Form 1040-ES), if not paying income tax through withholding or not paying sufficient income tax through withholding.
Calendar-year corporations: Pay the third installment of 2024 estimated income taxes, completing Form 1120-W for the corporation’s records.
Calendar-year S corporations: File a 2023 income tax return (Form 1120-S) and provide each shareholder with a copy of Schedule K-1 (Form 1120S) or a substitute Schedule K-1 if an automatic six-month extension was filed. Pay any tax, interest, and penalties due.
Calendar-year S corporations: Make contributions for 2023 to certain employer-sponsored retirement plans if an automatic six-month extension was filed.
Calendar-year partnerships: File a 2023 income tax return (Form 1065 or Form 1065-B) and provide each partner with a copy of Schedule K1 (Form 1065) or a substitute Schedule K1 if an automatic six-month extension was filed.
Employers: Deposit Social Security, Medicare and withheld income taxes for August if the monthly deposit rule applies.
Employers: Deposit nonpayroll withheld income tax for August if the monthly deposit rule applies.
September 30
Calendar-year trusts and estates: File a 2023 income tax return (Form 1041) if an automatic five-and-a-half-month extension was filed. Pay any tax, interest, and penalties due.
October 10
Individuals: Report September tip income of $20 or more to employers (Form 4070).