Most people think “tax season” starts in January and ends on April 15th (or October 15th if you’re waiting for those K-1s). But that’s filing season. It’s simply the administrative deadline for reporting what has already happened.

For founders, high-earning professionals and practice owners, the real tax season — and the only window where decisions still move real dollars — is Q4, October through December.

Many smart, successful people procrastinate tax planning because they feel like they still have time, or they assume their CPA can “fix it” in the spring. This is a fundamental misunderstanding of how the tax code works. Tax law is built on deadlines, and the most important one is the end of the “Tax Year” — December 31st.

Q4 is when you can still sign plan documents, strategically move money, harvest investment losses and lock in actions that legally and permanently lower what you owe. This is the period for action. If you wait until spring, the work becomes more about reporting than planning. You’ll find yourself paying a tax bill based on situations you could have changed — but didn’t.

Related: I Run a Portfolio of High-Growth Companies — This Practice Makes It All Possible

Q4 is when real tax strategy happens

Taxes feel expensive when they arrive as a surprise. The way to eliminate that sting is to treat Q4 like the closing sprint of your financial year. This is your last opportunity to look at your actual year-to-date data and strategically align your compensation, entity structure, retirement contributions and charitable giving.

This isn’t about checking off a list of isolated tactics. It’s an integrated process — each decision influences the others. For example, a large contribution to a new retirement plan reduces your taxable income, but it also affects your immediate cash flow. That cash flow position then influences your ability to prepay expenses or make a significant charitable gift. These are connected levers, and Q4 is your last chance to pull them in the right sequence to create the most favorable outcome.

What you can or cannot do is based on your source of income. Business owners, for instance, may be reimbursed based on their health insurance premiums via their W-2. Plus, they can contribute to their retirement accounts (401(k), IRAs, etc.) and conduct a Backdoor Roth Conversion (assuming they’re eligible and stay mindful of the pro-rata rule).

These business owners also make sure that any equipment, software or even vehicles for their businesses are purchased before the year is over.

In the case of high-income W-2 employees, the key optimization areas include maximizing 401(k) contributions (contribute a minimum of $23,500 in 2025, increasing to $24,500 in 2026), IRAs (contribute a minimum of $7,000 in 2025, increasing to $7,500 in 2026), HSA contributions, spending remaining FSA funds, completing a Backdoor Roth Conversion and performing tax-loss harvesting. Actual numbers depend on individual circumstances.

However, please note that the 401(k) contributions mentioned above refer only to employee limits. In 2025, the limit on contributions (both employee and employer) is $70,000, which goes up to $72,000 in 2026.

It’s equally important that business owners set up their discretionary plans, such as Solo 401(k)s and profit-sharing plans, by December 31st. In many cases, you may fund these plans later — up to your tax filing deadline, including extensions — but the plan itself must exist this year. Model the numbers now, sign on time and your 2025 tax bill drops. As a side benefit, lenders prefer compensation patterns and retirement plan funding that appear deliberate rather than reactive.

Next, you must coordinate capital gains, losses and equity compensation. And if you sold equity, harvested your portfolio or expect to exercise your stock options, this year, Q4, is your last chance to control which year gets the income. Perhaps the most well-known strategy is harvesting losses to offset gains realized in the year. Just be aware that if you repurchase the “same” security within 30 days, your deduction will be prohibited.

For founders and executives, Q4 is your last chance to decide whether to exercise non-qualified stock options (NSO) this year without pushing yourself into a higher bracket. Run an Alternative Minimum Tax (AMT) check before exercising incentive stock options (ISO) to see whether doing so would trigger AMT.

Charitable giving is part of this effort, too. If philanthropy is part of your plan, Q4 is how you maximize its impact. Never donate cash if you have appreciated stock held more than one year. By donating appreciated stock directly, you deduct the full fair market value and permanently avoid capital gains tax. If you plan to “bunch” deductions, a Donor-Advised Fund (DAF) is the ideal tool. Contribute a large amount in Q4, take the deduction this year, and direct grants to charities over the coming years.

Finally, use Q4 to ensure your books are clean and ready for year-end tax estimates. This is one of your last opportunities to avoid surprises and understand your tax liability accurately. If you wait until March to reconcile, you’re guessing. In December, you still have time to make data-driven tax moves instead of shooting in the dark. Good books don’t just make filing easier — they enable better planning because your strategy is based on reality.

Related: 4 Tax Strategies Every High-Earning Entrepreneur Needs to Know for 2025

Filing season won’t fix what you missed

The most common tax regret I hear isn’t about a specific number — it’s about lack of proactiveness. It’s the sinking feeling people get in March when they realize they assumed they had more time than they did.

At filing time, your CPA is a historian, not a time traveler. In many cases, no matter how much they want to help, they can’t apply tax-saving strategies retroactively in April for the previous year (with a few exceptions).

While some actions can be taken after year-end — like late S-Corp elections (when applicable), employer-side retirement contributions and certain IRA contributions — many strategies are permanently closed once the calendar turns. You cannot adopt a retirement plan you never established. You cannot execute a charitable transfer you didn’t make. You cannot harvest losses after the market closes on December 31st.

Those doors are locked.

The myth is that you can “optimize at filing.”
The truth is that you optimize in Q4, then file what happened.

Your next tax year starts now

The fastest way to improve your 2026 outcome is to treat these last weeks of 2025 as the start of a new playbook, not the end of an old one. Q4 is the best time to clean up your books, review your compensation and course-correct based on the year that just happened.

Schedule a mandatory tax strategy session with your tax advisor. Review your actual year-to-date revenue and margins. Decide whether to accelerate or defer income (if possible). Finalize payroll and owner distributions. If you run a practice, use this period to confirm that your entity and compensation structures still make sense after a year of growth, hiring, or operational changes.

The message is simple: Treat taxes as a year-round system, not an April fire drill, and you’ll spend less while staying in control.

April is when you report. December is when you decide.

Use Q4 to sequence the right moves, and you’ll keep more, document better and enter the new year with a plan instead of a pile of receipts.