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).