September 10
Individuals: Reporting August tip income, $20 or more, to employers (Form 4070).
September 15
Individuals: Paying the third installment of 2025 estimated taxes, if not paying income tax through withholding (Form 1040-ES).
Corporations: Paying the third installment of 2025 estimated income taxes.
S Corporations: Making contributions for 2024 to certain employer-sponsored retirement plans, if an automatic six-month extension was filed.
Partnerships: Filing a 2024 income tax return (Form 1065 or Form 1065-B), if an automatic six-month extension was filed.
Tax Exempt Orgs: Deposit Estimated Tax for 3rd quarter due on Unrelated Business Taxable Income for Tax-Exempt Organizations. Use Form 990-W to determine the amount of estimated tax payments required.