Key tax dates and to-do list: September 2026

September is one of the busiest compliance months of the year, anchored by two deadlines that dominate the calendar: September 15 for extended returns for calendar-year S-corporations (Form 1120-S, U.S. Income Tax Return for an S Corporation, and partnerships (Form 1065, U.S. Return of Partnership Income), along with Q3 2026 estimated tax payments for individuals, corporations, and trusts.

For many firms, September 15 rivals April 15 in intensity, since it’s the final deadline for pass-through entities that extended in March, as well as any K-1s generated that day immediately become inputs for the individual returns still on extension until October 15. There are no major new extension deadlines beyond that single date, but the downstream effects ripple through the rest of the month as K-1 packages go out to individual clients and their preparers.

The weeks leading up to the September 15 should be spent finalizing the entity returns that are actually ready, formally extending (via Form 7004, if not already done) any entity that genuinely won’t make the deadline, and making sure staff aren’t burning hours chasing documents from clients who have gone quiet. Once the 15th passes, attention shifts immediately to distributing K-1s and flagging which individual returns now depend on them, since it’s only a short time until October 15. This is also a good month to review your firm’s fall staffing plan, since the individual extension deadline is only a month away and you’ll want capacity mapped out before it arrives.

Along the way, be sure to pay attention to these important IRS and compliance deadlines.

In September, it is recommended that you:

• File all extended S-corp and partnership returns. September 15 is the hard deadline. Confirm all extended pass-through returns are completed and submitted.
• Run retirement plan reviews for business owner clients. September is ideal for cash balance plans and defined benefit plan proposals. Contributions must be made by the return due date, including extensions, but the plan must be established before year-end.

Key Compliance Dates

Thursday, September 10: Tips reporting for August (Form 4070).

Monday, September 14: Regular-method excise taxes deposit for the last 16 days of August.

Tuesday, September 15: Third installment of 2026 estimated tax due for individuals and calendar-year corporations. S corporation (1120-S) and Partnership (1065) returns due if on extension. Monthly payroll tax deposit for August (Form 941).

Friday, September 25: Communications and air transportation taxes deposit for the last 16 days of August.

Tuesday, September 29: Special September deposit rule: Regular-method excise taxes deposit for the period beginning September 16 and ending September 26. 

Wednesday, September 30: Wagering Tax (Form 730) and Heavy Highway Vehicle Use Tax (Form 2290) for August due. Final due date for special September deposit rules for excise tax.

Intuit® Accountant Resources

• Key Tax Facts for 2026 and Key Tax Facts for 2025
• The September 2026 calendar with filing deadlines and reminders for you and your clients.
• Tax update for TY26: Navigating the OB3 Act and more
• One Big Beautiful Bill summary and tax changes
• Navigating the tax landscape
• IRS annual inflation adjustments for tax years 2025 and 2026