September 15
Individuals – Paying the third installment of 2023 estimated taxes, if not paying income tax through withholding (Form 1040-ES).
Calendar-Year Corporations – Paying the third installment of 2023 estimated income taxes.
Calendar-Year S Corporations – Filing a 2022 income tax return (Form 1120-S) and paying any tax, interest, and penalties due if an automatic six-month extension was filed.
Calendar-Year S Corporations – Making contributions for 2022 to certain employer-sponsored retirement plans if an automatic six-month extension was filed.
Calendar-Year Partnerships – Filing a 2022 income tax return (Form 1065 or Form 1065-B) if an automatic six-month extension was filed.
October 2
Trusts and estates – Filing an income tax return for the 2022 calendar year (Form 1041) and paying any tax, interest, and penalties due if an automatic five-and-a-half-month extension was filed.
Employers – Establishing a SIMPLE or a Safe-Harbor 401(k) plan for 2022, except in certain circumstances.
October 10
Employees Who Work for Tips – Reporting September tip income of $20 or more to employers (Form 4070).