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Key September 2026 Tax Deadlines for Businesses

September 2026 introduces a wave of critical tax deadlines for business owners and entities. Whether you are finalizing extended returns for an S corporation or partnership, managing corporate estimated tax payments, or depositing employer payroll taxes, staying ahead of these dates is essential for keeping your financial filings on track and avoiding penalties.

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September 15 Business Tax Due Dates

S Corporations (Form 1120-S)

If your S corporation requested an automatic six-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any outstanding 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 valid substitute. If applicable, Schedule K-3 (Form 1120-S) must also be distributed.

Partnerships (Form 1065)

Similarly, partnerships that timely requested an automatic six-month extension must file their 2025 calendar year return (Form 1065). Partners must be provided with their Schedule K-1 (Form 1065) or a substitute, along with Schedule K-3 (Form 1065) where applicable.

Corporations

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

Employer Payroll and Withholding Taxes

If your business is subject to the monthly deposit rules, you must make your deposit for Social Security, Medicare, and withheld income tax for the month of August 2026. This is also the required due date for any nonpayroll withholding deposits for August under the monthly rule.

September 30 Deadline for Estates and Trusts

Fiduciaries of estates and trusts who were granted a 5½-month extension must file their 2025 calendar year return (Form 1041) by September 30. You must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or an approved substitute if applicable.

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Exceptions for Weekends, Holidays, and Disaster Areas

Standard filing dates can occasionally shift. If a scheduled due date happens to fall on a Saturday, Sunday, or legal holiday, your deadline is automatically extended to the next business day that is not a legal holiday.

Furthermore, businesses located in geographically designated disaster areas may be eligible for extended due dates. To determine if your specific area qualifies for federal tax relief and to find updated filing extension dates, consult the official resources below:

Stay Ahead of Your Fall Tax Obligations

Missing a filing or deposit deadline can lead to unnecessary stress and potential financial penalties. If you need assistance finalizing your extended S corporation or partnership returns, or if you want to ensure your payroll and estimated tax deposits are calculated correctly, reach out to schedule a consultation. We can help you navigate these September compliance requirements with confidence.

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