Essential September 2026 Tax Deadlines for Your Business

September brings several important tax deadlines for businesses, especially if you filed an extension earlier in the year or if you manage a growing team. From extended returns for S corporations and partnerships to corporate estimated tax payments and employer payroll deposits, reviewing these key due dates will help keep your business filings and payments on track without added stress.

Small business owner reviewing tax documents

September 15 — S Corporations

If you requested an automatic six-month extension for your S corporation, your 2025 calendar year income tax return (Form 1120-S) is due. You must also pay any remaining tax due at this time. Additionally, ensure you provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute, along with Schedule K-3 (Form 1120-S) if applicable.

September 15 — Corporations

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

September 15 — Partnerships

Did your partnership timely request an automatic six-month extension? If so, your 2025 calendar year return (Form 1065) is now due. Just like S corporations, you need to provide each partner with their Schedule K-1 (Form 1065) or a substitute, and Schedule K-3 (Form 1065) where applicable.

September 15 — Payroll and Withholding Taxes

If you are an employer operating under the monthly deposit rules, you must make your deposit of Social Security, Medicare, and withheld income tax for August 2026. This is also the deadline to deposit nonpayroll withholding for August 2026 under the monthly deposit rule.

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September 30 — Fiduciaries of Estates and Trusts

For fiduciaries who were granted a 5½-month extension, your 2025 calendar year return (Form 1041) is due at the end of the month. If applicable, be sure to provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute.

Weekend, Holiday, and Disaster Area Rules

Missing a deadline is a common worry, but standard rules exist for dates that fall outside regular business days. If a 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 a legal holiday.

Furthermore, if your business is located in a geographic area designated as a disaster area, your due dates will be extended. You can check if your area qualifies for tax relief by visiting these resources:

Keeping Your Filings Compliant and Stress-Free

Navigating extended tax deadlines and calculating estimated payments can feel overwhelming, especially when you are busy running your daily operations. If this sounds familiar, we can walk you through it step by step. Reach out to schedule a consultation, and we will make sure your business filings are accurate and submitted on time.

Virtual AI
If you’re ready to get a handle on your tax situation, reach out and we’ll guide you through each step.
Let’s Sort This Out
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