As summer winds down, an important tax deadline is approaching for many individuals, self-employed professionals, and business owners. September 15 marks the third estimated tax payment deadline of the year and the extended filing deadline for certain business tax returns. 

Understanding what is due and taking time to review your tax position can help you stay on track and avoid surprises as the end of the year approaches. 

Third-Quarter Estimated Tax Payments 

For individuals who are required to make estimated tax payments, the third payment for the 2026 tax year is due September 15. 

Estimated taxes generally apply to people who receive income that is not subject to sufficient tax withholding. This can include self-employed individuals, business owners, partners, S corporation shareholders, investors, and others with significant non-wage income. 

Unlike employees who typically have taxes withheld from each paycheck, these taxpayers may need to make payments throughout the year to cover their expected federal tax liability. 

The IRS divides estimated tax payments into four payment periods. For 2026, the deadlines are April 15, June 15, September 15, and January 15, 2027. 

Why Estimated Payments Matter 

Federal income taxes operate on a pay-as-you-go system. Waiting until you file your annual tax return to pay the entire amount you owe can potentially result in an underpayment penalty. 

It is also important to remember that estimated payments do not necessarily remain the same throughout the year. Changes in income, business performance, investment gains, deductions, or other circumstances can affect your expected tax liability. 

September can therefore be a useful opportunity to review what you have earned and paid so far and determine whether your remaining estimated payments should be adjusted. 

Extended Business Tax Returns Are Also Due 

September 15 is also an important filing deadline for many businesses that requested an extension earlier in the year. 

Calendar-year partnerships that received a six-month extension generally must file Form 1065 by September 15. Calendar-year S corporations that received an extension generally must file Form 1120-S by the same date. 

An extension provides additional time to file a return, but it does not necessarily extend the time to pay taxes that were originally due. Businesses approaching the extended deadline should make sure their records and required documentation are complete and work with their tax professionals to address any outstanding filing requirements. 

Use the September Deadline as a Tax Planning Checkpoint 

September 15 is more than another date on the tax calendar. With several months remaining in the year, it provides an opportunity to look at the bigger financial picture. 

Business owners and individuals with variable income may want to review year-to-date earnings, estimated payments, investment activity, business results, and any significant financial changes that could affect their 2026 taxes. 

Addressing these issues now can provide more time to make informed decisions before year-end rather than waiting until tax filing season. 

If you have questions about your estimated tax payments, extended business return, or year-end tax planning, reach out to Adeptus to discuss your individual situation.