September 2026 Business Tax Due Dates: Ensure Your Filings Are on Track

September brings several crucial tax deadlines for businesses and entities. Whether you operate an S corporation, manage a partnership, or handle employer payroll responsibilities, this month carries key requirements for extended returns, estimated tax installments, and standard deposits.

Review the essential September 2026 due dates outlined below to help keep your business filings and tax payments on track, preventing unnecessary interest or penalties.

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September 15: Corporate, Partnership, and Payroll Deadlines

S Corporations

If your S corporation requested an automatic six-month extension, September 15 is the deadline to file your 2025 calendar year income tax return (Form 1120-S) and pay any outstanding tax due. You must also provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable, Schedule K-3 (Form 1120-S) or its substitute must be provided as well.

Corporations

For calendar year C corporations, September 15 marks the deadline to deposit the third installment of estimated income tax for the 2026 tax year.

Partnerships

Partnerships that timely requested an automatic six-month extension must file their 2025 calendar year return (Form 1065) by this date. Each partner must be provided with a copy of their Schedule K-1 (Form 1065) or a substitute, along with Schedule K-3 (Form 1065) or a substitute if applicable.

Social Security, Medicare, and Withheld Income Tax

If you are an employer subject to the monthly deposit rules, you must make your deposit of Social Security, Medicare, and withheld income tax for the month of August 2026 by September 15. This is also the due date for the nonpayroll withholding deposit for August 2026 under the monthly deposit rule.

September 30: Fiduciaries of Estates and Trusts

Fiduciaries who were granted a 5½-month extension must file their 2025 calendar year return (Form 1041) by September 30. Where applicable, you must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a valid substitute.

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Special Rules for Weekends, Holidays, and Disasters

Weekend and Holiday Adjustments

If a tax due date happens to fall on a Saturday, Sunday, or legal holiday, the deadline is automatically extended until the next business day that is not a legal holiday.

Disaster Area Extensions

When a specific geographical area is designated as a disaster area, filing and payment due dates will generally be extended. To verify whether a particular area has been designated a disaster area and to review updated filing extension dates, visit the following federal websites:

Secure Your Fall Tax Compliance

Properly managing extended returns and regular deposits is fundamental to maintaining your company's financial health. If you have questions about these September filing requirements, need assistance generating Schedule K-1s, or want to review your estimated tax calculations, our team is ready to help. Schedule a consultation today to ensure your tax obligations are met accurately and on time.

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