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Essential September 2026 Tax Deadlines 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 routine employer payroll deposits. Whether your business operates locally in Maryland, Virginia, or the District of Columbia, or you serve customers nationwide, staying compliant means keeping a close eye on the calendar. Review the key September 2026 due dates below to help ensure your business filings and tax payments remain right on track.

Business Operations and Deadlines

September 15: S Corporations

For S corporations that previously 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 tax due. Additionally, you must provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable to your business, you must also furnish Schedule K-3 (Form 1120-S) or its substitute.

September 15: Corporations

Calendar year corporations must deposit their third installment of estimated income tax for the 2026 tax year by this date.

September 15: Partnerships

If your partnership timely requested an automatic six-month extension, you must file your 2025 calendar year return (Form 1065) by September 15. You are also required to provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute. As with S corporations, you must provide Schedule K-3 (Form 1065) or a substitute Schedule K-3, if applicable.

September 15: Payroll and Nonpayroll Withholding Deposits

For employers subject to the monthly deposit rules, September 15 is the deadline to deposit Social Security, Medicare, and withheld income taxes for August 2026. This is also the due date for nonpayroll withholding deposits for August 2026, provided the monthly deposit rule applies to your organization.

September 30: Fiduciaries of Estates and Trusts

If you were granted an extension of 5½ months, you must file your 2025 calendar year return (Form 1041) by September 30. Where applicable, you must also furnish each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Weekend and Holiday Rule

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

Disaster Area Extensions

If your business is located in a geographical region designated as a disaster area, your tax due dates will be extended. To verify whether your area qualifies for a disaster designation and to confirm any revised filing extension dates, you can check the following official resources:

Professional Tax Guidance for the Fall Deadline Season

Managing extended returns and estimated payments correctly is a vital part of minimizing your tax liability and protecting your business infrastructure. If you need assistance navigating these September deadlines, or managing broader federal and state compliance requirements, PM Enterprises Inc is here to help. Reach out to LLoyd Mallory and our top-rated team today to ensure your business tax strategies are aligned and fully compliant.

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