September 2026 Tax Deadlines for Businesses

September brings several essential tax deadlines for businesses operating in Braintree, Quincy, and the greater Boston area. From extended returns for S corporations and partnerships to corporate estimated tax payments and employer payroll tax deposits, staying compliant requires careful attention to the calendar. Review the key due dates below to help keep your small business filings, bookkeeping, and payments firmly on track as we move into the fall.

September 15: S Corporation Extended Returns

If you requested an automatic six-month extension for your S corporation, September 15 is the deadline to file your 2025 calendar year income tax return (Form 1120-S) and pay any tax due. 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 to your business operations, you must also furnish Schedule K-3 (Form 1120-S) or a substitute.

September 15: Corporate Estimated Taxes

Calendar year corporations must deposit their third installment of estimated income tax for the 2026 tax year by September 15.

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September 15: Partnership Extended Returns

Partnerships that timely requested an automatic six-month extension are required to 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 Schedule K-1. Where applicable, Schedule K-3 (Form 1065) or a substitute must also be distributed.

September 15: Payroll and Withholding Tax Deposits

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

One Accounting Tax® Since 2017
Call/Text: (617) 829-0928 or email service@oneaccountingtax.com to schedule an in-person consultation or video call with our Tax Advisors (IRS Enrolled Agent, EA) today. Serving Braintree, Quincy, and Greater Boston with full-service accounting—tax preparation, payroll, bookkeeping, and year-round tax planning.
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September 30: Fiduciaries of Estates and Trusts

Fiduciaries must file a 2025 calendar year return (Form 1041) by September 30, provided you were given an extension of 5½ months. If applicable to the estate or trust, 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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Adjustments for Weekends, Holidays, and Disaster Areas

When any tax 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.

Please note that when a geographical area is designated as a disaster area, due dates will be extended. To verify whether a specific region has been designated a disaster area and to check for filing extension dates, visit the following websites:

Partner With a Greater Boston Tax Professional

Managing strict filing deadlines on top of daily operations can be demanding for any small business or real estate investor. Missing an extended return or a sales and meals tax filing can lead to costly penalties and IRS auditing complications.

Whether you need an Accountant, Tax Preparer, or an IRS Enrolled Agent (EA) to handle complex filings, our team is here to help. Contact us today to schedule a consultation and ensure your business tax preparation, bookkeeping, and payroll remain fully compliant year-round.

One Accounting Tax® Since 2017
Call/Text: (617) 829-0928 or email service@oneaccountingtax.com to schedule an in-person consultation or video call with our Tax Advisors (IRS Enrolled Agent, EA) today. Serving Braintree, Quincy, and Greater Boston with full-service accounting—tax preparation, payroll, bookkeeping, and year-round tax planning.
Contact Our Local Tax Advisors Today!
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