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Essential September 2026 Tax Deadlines for Businesses

September brings a wave of critical tax deadlines for business owners, encompassing everything from extended returns for S corporations and partnerships to corporate estimated tax payments and employer payroll deposits. At Golden State Tax & Business Services, we believe proactive planning is the key to maintaining compliance and minimizing tax friction. To help you manage cash flow and avoid unwelcome surprises, we have compiled the key filing and payment due dates you need to keep your business on track this month.

Business team in a planning meeting

September 15: S Corporation Deadlines

If you operate as an S corporation and requested an automatic 6-month extension, you must file your 2025 calendar year income tax return (Form 1120-S) and pay any tax due by this date. Additionally, you are required to provide each shareholder with a copy of their Schedule K-1 (Form 1120-S) or a substitute Schedule K-1. If applicable, you must also provide Schedule K-3 (Form 1120-S) or a substitute Schedule K-3 to the relevant shareholders.

September 15: Corporate Estimated Taxes

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

September 15: Partnership Returns

Partnerships that timely requested an automatic 6-month extension must file their 2025 calendar year return (Form 1065) by this date. Similar to S corporations, you must provide each partner with a copy of their Schedule K-1 (Form 1065) or a substitute Schedule K-1. If applicable to your partnership, you must also issue Schedule K-3 (Form 1065) or a substitute Schedule K-3 to your partners.

September 15: Payroll and Withholding Deposits

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

If this made you think, “I should probably ask someone,” that’s us.
A quick conversation can clarify whether this actually applies to you—and whether there’s an opportunity you shouldn’t ignore. General guidance is helpful, but smart decisions come from advice tailored to your numbers. Whether now or later, we’re happy to help you plan ahead.
GET IN TOUCH WITH US

September 30: Fiduciaries of Estates and Trusts

Fiduciaries who were granted an extension of 5½ months must file their 2025 calendar year return (Form 1041) by the end of the month. If applicable, you are also required to provide each beneficiary with a copy of their Schedule K-1 (Form 1041) or a substitute Schedule K-1.

Standard Weekend and Holiday Rules

As a general rule, if a standard tax due date happens to fall on a Saturday, Sunday, or legal holiday, the deadline is automatically extended until the next business day that is not itself a legal holiday.

Disaster Area Tax Relief

Please note that when a geographical area is designated as a disaster area, the IRS will generally extend affected due dates. To verify whether your specific region has been designated as a disaster area and to review any updated filing extension dates, visit the following resources:

Stay Compliant and Plan Ahead

Navigating these deadlines efficiently allows you to spend less time worrying about compliance and more time focusing on your business. Led by Ryan Shull and our team of experts in Rocklin, California, Golden State Tax & Business Services leverages modern technology and deep expertise to help closely held business owners make confident financial decisions year-round.

If you need assistance finalizing an extended return or want to discuss multi-year tax strategies to reduce your future liabilities, schedule a consultation with our advisory team today.

If this made you think, “I should probably ask someone,” that’s us.
A quick conversation can clarify whether this actually applies to you—and whether there’s an opportunity you shouldn’t ignore. General guidance is helpful, but smart decisions come from advice tailored to your numbers. Whether now or later, we’re happy to help you plan ahead.
GET IN TOUCH WITH US
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