September may feel far removed from tax season, but millions of Americans have an important date approaching fast. The September 15 tax deadline affects many self-employed workers, freelancers, investors, business owners, and others who need to make estimated tax payments. It is also a major filing deadline for certain partnerships and S corporations that received extensions earlier this year. Missing the date could mean penalties, interest, or an unpleasant surprise when the next tax bill arrives. Here is what taxpayers should know before September 15, 2026.
