October is not an early start. It is the last point at which a business owner still has enough of the year left to change the outcome. By December, most year-end moves are already locked in by deadlines that fall before the calendar flips. This article is general information, not tax advice for a specific situation, and filing dates shift year to year and should be confirmed for the current tax year.
Table of Contents
- Why is December too late to make year-end tax decisions?
- What can you actually still change in October that you cannot change in December?
- How does a calendar-year business actually structure a year of tax planning?
- Does the October rule apply if your business does not use a December 31 year end?
- Which year-end strategies have deadlines that fall after December 31?
- What goes wrong when owners wait until December?
- What should a business owner do in October to make December easier?
- Key Takeaways
- References
Why is December too late to make year-end tax decisions?
December gives you eleven months of actual data and roughly three weeks to act on it. October gives you eight months of actuals and a full quarter of runway, according to My CIWM. That difference in remaining time changes what is possible.
Tax preparation reports activity that already happened. Tax planning is forward-looking, so it needs time left on the clock to be worth anything, as SMG ABA explains. A planning conversation in December is mostly a review of what you cannot change.
Several year-end moves are gated by hard dates that land before December 31, not on it. A Solo 401(k) plan must exist before the calendar year closes, even though funding can wait until the filing deadline, according to My CIWM. Missing that date removes the entire strategy for the year.
Once December arrives, most owners are closing books, not weighing entity or retirement decisions. The value of a planning conversation is proportional to how much of the year is still changeable. By December, that value has largely expired.
What can you actually still change in October that you cannot change in December?
Retirement plan adoption is the clearest example. A Solo 401(k) must be established by December 31 of the year you want contributions to count for, even though funding can wait until the filing deadline, according to My CIWM. October leaves time to evaluate the plan, open it, and structure contributions. December leaves almost none.
Pre-tax contributions reduce taxable income at your marginal bracket. The same contribution is worth more or less depending on the bracket it offsets. A $10,000 pre-tax retirement contribution saves up to $2,400 in federal income taxes for someone in the 24% bracket, and up to $3,200 for someone in the 32% bracket, according to My CIWM.
Capital loss harvesting has to clear the wash-sale rule. That rule disallows a loss if the same or substantially identical security is bought back within 30 days before or after the sale, according to My CIWM. The 30-day window needs calendar room, which December does not provide.
Up to $3,000 of excess capital losses can offset ordinary income in a year, with the rest carried forward, so the sequencing decision matters. Estimated tax timing also favors October. The Q3 payment was due September 15 and Q4 lands January 15, so October is when the fourth-quarter estimate is still adjustable, according to My CIWM.
How does a calendar-year business actually structure a year of tax planning?
A workable sequence runs monthly bookkeeping, a midyear review, a focused September–October review, November strategy evaluation, December completion of year-end decisions, and post-year-end filings, according to SMG ABA. Each stage has a distinct job.
The September–October review is where the year's numbers are solid enough to trust and the remaining months are still flexible. That is the window where planning actually happens. Businesses weighing a major transaction, ownership change, or significant investment should start earlier than September or October, because those moves take longer to structure, as SMG ABA notes.
Monthly books are the input that makes any of this possible. Planning off stale or incomplete numbers just produces guesses. The point of the cadence is that no single decision gets made under time pressure. A year of tax planning is a series of small, deliberate moves, not a December scramble.
Does the October rule apply if your business does not use a December 31 year end?
No. The October rule is a shorthand for calendar-year businesses. The IRS defines a fiscal year as 12 consecutive months ending on the last day of any month except December, according to the IRS. A 52-53-week tax year is treated as a fiscal tax year under IRS rules.
Fiscal-year businesses should plan relative to their own tax-year end rather than December 31. The same logic applies, shifted to the right date, as SMG ABA explains. The principle is not October specifically. It is roughly one full quarter of remaining, changeable year.
Owners who inherited a non-calendar year end often do not realize their planning calendar is different from their peers'. They watch other owners make moves in October and assume they are behind, when their own planning window sits elsewhere on the calendar.
Which year-end strategies have deadlines that fall after December 31?
SEP IRA contributions can be made as late as the tax filing deadline including extensions, with contributions up to 25% of net self-employment income and a 2026 maximum of $72,000, according to My CIWM. The funding deadline sits after year end, but the decision to use a SEP IRA still benefits from October planning.
Solo 401(k) funding can also wait until the filing deadline, but the plan document itself must exist before the calendar year closes. The asymmetry catches people. For 2026, the Solo 401(k) employee contribution tops out at $24,500, with an employer profit-sharing contribution up to 25% of compensation and a combined maximum of $72,000, according to My CIWM.
The combined maximum rises to $80,000 with catch-up for ages 50–59 and 64+, and $83,250 with catch-up for ages 60–63, according to My CIWM. A Solo 401(k) fits self-employed individuals with no full-time employees other than a spouse.
Because some deadlines sit after year end, the decision to use them still has to be made before year end. That is the October work: choosing the strategy, confirming eligibility, and setting the plan in motion while there is still time to adjust.
What goes wrong when owners wait until December?
The pattern is consistent: decisions get made with incomplete information and no time to reverse them. Common casualties are retirement plan adoption, loss harvesting blocked by the wash-sale window, and fourth-quarter estimated payments set without a real projection, according to My CIWM.
Owners who are already behind on filings face a compounded version of the problem. Catch-up work has to be sequenced before current-year planning is useful. A business carrying unfiled back returns cannot plan cleanly against a year that is not yet closed on paper.
This is the work Spencer Accounting Group does. The firm handles multi-state sales tax resolution, Strategic Tax Planning, Bookkeeping, and Tax Filing, plus Back Tax Return Filing for owners who are behind. All of it is handled virtually from anywhere, with no office visit required.
What should a business owner do in October to make December easier?
Pull year-to-date financials and confirm the books are actually closed through September. Project the last quarter so the full-year income picture is visible before decisions get made. These two steps turn planning from a guess into a calculation.
List every move with a hard date attached: plan adoption, contribution funding, estimated payments, loss harvesting. Note which dates fall before December 31 and which fall after. The list makes the sequence visible.
Estimate the marginal bracket the year will land in, since that determines what a pre-tax contribution is actually worth. Confirm the current year's filing dates rather than relying on last year's calendar, because dates shift year to year. October is the month to make December boring.
Key Takeaways
- October leaves eight months of actual data and a full quarter to act; December leaves eleven months of data and about three weeks.
- Tax preparation looks backward at what happened; tax planning looks forward at what can still be changed.
- A Solo 401(k) plan must be established by December 31 of the year contributions are intended for, even though funding can wait until the filing deadline.
- SEP IRA contributions can be made as late as the tax filing deadline including extensions, up to 25% of net self-employment income with a 2026 maximum of $72,000.
- The wash-sale rule disallows a loss if the same or substantially identical security is repurchased within 30 days before or after the sale, so harvesting needs calendar room.
- The IRS defines a fiscal year as 12 consecutive months ending on the last day of any month except December, and fiscal-year businesses plan against their own year end.
- Filing dates shift year to year and should be confirmed for the current tax year; this article is general information, not tax advice for a specific situation.
References
- Why Year-End Tax Planning Should Start Before December — SMG ABA, 2026-08-20
- Year-End Tax Planning: Why Starting in September Beats Starting in December — My CIWM
- Tax years — Internal Revenue Service