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Key September 2026 Tax Due Dates for Businesses

September brings a crucial wave of tax deadlines for businesses, encompassing extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. Navigating these requirements accurately is essential to maintaining compliance and preventing unwanted scrutiny. Review the key due dates below to ensure your business filings and payments remain on track.

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Key September 15 Deadlines

S Corporations

If your S corporation requested an automatic 6-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any remaining tax due by September 15. Additionally, you are required to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1, along with Schedule K-3 (Form 1120-S) or a substitute Schedule K-3 if applicable.

Corporations

Calendar year corporations are required to deposit their third installment of estimated income tax for 2026.

Partnerships

Partnerships that timely requested an automatic 6-month extension must file their 2025 calendar year return (Form 1065). You must also supply each partner with a copy of their Schedule K-1 (Form 1065) or a substitute Schedule K-1, and, where applicable, Schedule K-3 (Form 1065) or a substitute Schedule K-3.

Social Security, Medicare, and Withheld Income Tax

For employers subject to the monthly deposit rules, September 15 is the deadline to deposit Social Security, Medicare, and withheld income tax for August 2026. This date also applies to the nonpayroll withholding deposit for August 2026 under the monthly deposit rule.

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September 30 Deadline for Estates and Trusts

Fiduciaries of estates and trusts must file a 2025 calendar year return (Form 1041) if they were granted a 5½-month extension. If applicable, ensure each beneficiary receives a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Rules for Weekends, Holidays, and Disaster Areas

Weekends and Holidays

Whenever a standard due date falls on a Saturday, Sunday, or legal holiday, the deadline is automatically extended to the next business day that is not a legal holiday.

Disaster Area Extensions

If your geographical area is designated as a disaster area, your due dates will be extended. To verify whether a specific region has received this designation and to check the corresponding filing extension dates, consult the following official resources:

Expert Resolution for Complex IRS Tax Problems

Missing deadlines can trigger severe penalties and complex compliance issues. At IRS Tax Pros, we do not handle routine bookkeeping or accounting—our sole focus is resolving intricate tax problems. As an Enrolled Agent, I have the federal authority to represent taxpayers directly before the IRS. If you have fallen behind on filings or face a challenging IRS issue, contact Sharon Morgan today to secure dedicated, expert representation.

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We solve tax problems for individuals and help tax pros solve tax problems for their clients.
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