Year-End 2026 Checklist: The September 15 Deadline and What Follows

Four dates close out the 2026 tax year, and three of them are non-negotiable.

September 15, 2026 — Q3 individual estimated tax payment. Also the extended due date for S-Corporation returns (Form 1120-S) and partnership returns (Form 1065). If your business filed an extension in March, this is the actual deadline, and the K-1s it produces are what your personal return depends on.

October 15, 2026 — Extended individual returns (Form 1040). The FBAR also carries an automatic extension to this date, with no request required.

December 31, 2026 — The cutoff for almost every planning decision that affects the 2026 return: retirement contributions to plans that require calendar-year funding, charitable gifts, loss harvesting, equipment placed in service, and entity elections.

January 15, 2027 — Q4 individual estimated tax payment.

The trap in this sequence is that September 15 and October 15 are filing deadlines for a year that is already closed, while December 31 is the last date you can still change the outcome. Most people spend the fall on the first two and let the third pass unexamined.

Most business owners pay Q3 on autopilot — one quarter of last year's tax, or whatever the software carried forward. That is safe harbor thinking, and safe harbor only protects you from penalties. It does not tell you what you will owe.

If 2026 has been materially better than 2025, safe harbor payments are building a balance due that lands in April. If it has been worse, you are lending the Treasury money at zero interest for six months. Either way the fix is the same: a real projection before you cut the check.

The projection needs your actual year-to-date income, a realistic Q4 forecast, and any one-time events — a property sale, a large vest, a distribution. Thirty minutes of arithmetic in September routinely changes the Q3 payment by five figures.

100% bonus depreciation is permanent for qualified property acquired after January 19, 2025. That removes the old year-end scramble driven by a phase-down schedule — but it does not remove the timing decision.

Property has to be placed in service by December 31 to be deducted in 2026, not merely ordered or paid for. Equipment sitting on a loading dock on January 2 belongs to the 2027 return. If a purchase is planned for early next year and 2026 is your higher-income year, moving it forward has real value; if 2027 looks stronger, the reverse is true.

Section 179 expensing runs in parallel, with a limit near $2.5 million (indexed) and a phase-out beginning around $4 million of total qualifying purchases. Section 179 is capped at business taxable income while bonus depreciation is not, so a business near breakeven should generally sequence bonus first.

For real estate placed in service this year, a cost segregation study is worth running before the return is filed rather than after.

Retirement contributions. A Solo 401(k) must generally be established by December 31 to accept 2026 employee deferrals, even though funding can follow later. A SEP-IRA can be both established and funded up to the extended return due date — which is why the establishment deadline, not the funding deadline, is the one that catches people. The 2026 limits: $24,500 elective deferral, $8,000 catch-up at 50, $11,250 at ages 60 through 63, and a $72,000 annual addition limit.

Charitable giving. The 0.5% AGI floor and the 35% cap on the benefit for top-bracket donors both changed the arithmetic this year. Bunching two years of giving into one, or routing a gift through a donor-advised fund, is materially more valuable than it used to be. For those 70 and a half or older, a qualified charitable distribution of up to $111,000 stays out of income entirely.

Loss harvesting. Realized losses offset realized gains without limit, and $3,000 of ordinary income beyond that. The wash sale rule requires a 30-day window on either side, so late December is genuinely late.

Roth conversions. A conversion has to happen by December 31 to count for 2026. The window is best in a year when income dipped — and that is a fact you know in November, not in April.

If you take one action from this list, make it the Q3 projection — it is the only item where September still changes the number.

After that, in order: confirm your extended business return is on track for September 15 and that the K-1s will arrive in time to file your personal return by October 15; check whether any foreign account crossed $10,000 at any point in 2026, which triggers the FBAR; and put a date on the calendar in November for the year-end planning conversation, while there is still time for the answers to matter.

The difference between a good tax year and an expensive one is almost never made in April. It is made in the ten weeks between now and December 31.