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September 2026 Business Tax Due Dates

September brings several critical tax deadlines for business owners, including extended returns for S corporations and partnerships, corporate estimated tax payments, and employer payroll tax deposits. Reviewing these key due dates now can help ensure your business filings and payments remain on track.

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September 15: S Corporation Extended Returns

If you requested an automatic six-month extension for your S corporation, your 2025 calendar year income tax return (Form 1120-S) is due on September 15, along with any tax owed. By this date, 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, ensure you also supply Schedule K-3 (Form 1120-S) or its substitute.

September 15: Corporate Estimated Taxes

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

September 15: Partnership Extended Returns

Partnerships that timely requested an automatic six-month extension must file their 2025 calendar year return (Form 1065) by September 15. Additionally, partnerships must provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute, along with Schedule K-3 (Form 1065) or its substitute, if applicable.

September 15: Payroll and Withholding Deposits

If you are an employer 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 serves as the deadline for August 2026 nonpayroll withholding deposits 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. If applicable, you must also provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

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Weekend and Holiday Rules

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

Disaster Area Tax Extensions

When a specific geographical area is designated as a disaster area, tax due dates are generally extended to provide relief. To determine whether your area has received a disaster designation and to check the updated filing extension dates, visit the following resources:

Stay Ahead of Your Fall Tax Obligations

Managing multiple filing extensions and deposit schedules can be demanding, but meeting these deadlines is essential for staying compliant. If you need assistance preparing your extended returns or ensuring your estimated payments are on track, reach out to schedule a consultation with our tax professionals.

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