The December Tax Playbook: High-Impact Moves to Make Before the Calendar Turns
By the time most people think seriously about taxes, the opportunity to influence the outcome has already passed. Tax planning done in February or March is largely a documentation exercise — you are recording history, not shaping it. December is different. For the next several weeks, the tax code remains malleable in ways it will not be once January arrives.
This guide is organized around two categories: moves that must happen before December 31, and preparatory steps that can extend into early January or tax season without losing their value. Knowing the difference matters as much as knowing the strategies themselves.
Moves That Must Happen Before December 31
1. Accelerate Deductions If You Expect Lower Income Next Year
The logic here is straightforward: a deduction is worth more in a year when your marginal tax rate is higher. If you anticipate earning less in 2025 — due to a job change, retirement, or a business slowdown — pulling deductible expenses into 2024 locks in a larger tax benefit.
For individuals, this might mean prepaying your January mortgage installment in late December (the interest portion is deductible in the year paid), scheduling a medical procedure you have been deferring, or making a charitable contribution you planned for early next year.
For business owners, it means reviewing outstanding vendor invoices and paying them now, purchasing supplies and materials before year-end, or investing in software subscriptions that renew annually.
2. Harvest Investment Losses
If your taxable brokerage account holds positions that have declined in value, selling them before December 31 allows you to realize capital losses that offset capital gains earned elsewhere in the portfolio. If your losses exceed your gains, up to $3,000 of the excess can offset ordinary income, with the remainder carried forward to future years.
The rule to watch is the wash-sale rule: repurchasing a substantially identical security within 30 days before or after the sale disallows the loss. You can immediately reinvest in a similar — but not identical — position to maintain your market exposure while preserving the tax benefit.
Who this applies to: Anyone with a taxable investment account who has realized gains or holds positions with unrealized losses.
3. Maximize Retirement Contributions
For employees, 401(k) contributions for 2024 must be made through payroll by your last paycheck of the year. The 2024 limit is $23,000, with an additional $7,500 catch-up contribution available for those 50 and older. If you are not on track to hit your limit, contact your HR or benefits administrator now — payroll changes often require lead time.
For self-employed individuals and business owners, the contribution deadline for a Solo 401(k) is December 31 for employee elective deferrals, though employer contributions can generally be made up to the tax filing deadline including extensions. SEP-IRA contributions can be made until the filing deadline as well, making them a flexible option even for those who have not yet set up an account.
Who this applies to: Anyone with earned income who has not maxed out tax-advantaged retirement accounts.
4. Bunch Charitable Contributions
The 2024 standard deduction is $14,600 for single filers and $29,200 for married couples filing jointly. For many households, itemized deductions — including charitable gifts — fall just below those thresholds, meaning they receive no additional tax benefit from their generosity.
Bunching solves this by concentrating two or more years of charitable giving into a single calendar year, pushing total itemized deductions above the standard deduction threshold. A donor-advised fund (DAF) is the most flexible vehicle for this approach: you contribute a lump sum in December, claim the full deduction this year, and distribute grants to individual charities on your own timeline.
Who this applies to: Households whose itemized deductions in any single year fall below the standard deduction but whose two-year total would exceed it.
5. Business Equipment Purchases and Section 179
Section 179 of the tax code allows businesses to deduct the full cost of qualifying equipment and software in the year it is placed in service, rather than depreciating it over several years. For 2024, the deduction limit is $1,220,000, with a phase-out beginning at $3,050,000 of total qualifying purchases.
Bonus depreciation — currently at 60 percent for 2024 after the phase-down from 100 percent — applies to new and used qualifying property and stacks with Section 179 in some circumstances.
The key requirement: the asset must be placed in service before December 31. Ordering equipment in December but not receiving or using it until January does not qualify.
Who this applies to: Business owners who need equipment, vehicles, or technology and have taxable income to offset.
6. Review and Adjust Withholding or Make a Q4 Estimated Payment
If you have had significant income events in 2024 — a bonus, a property sale, a large freelance project, or a Roth conversion — your existing withholding may be insufficient. Filing with a large balance due is expensive; the underpayment penalty currently runs approximately 8 percent annually on the shortfall.
The Q4 estimated tax payment is due January 15, 2025, but reviewing your situation in December gives you time to calculate an accurate figure and avoid surprises. W-2 employees can also submit a revised Form W-4 to have additional tax withheld from remaining paychecks.
Moves That Can Extend Into Early 2025
7. Fund a Health Savings Account (HSA)
If you are enrolled in a qualifying high-deductible health plan, contributions to an HSA can be made up to the tax filing deadline (April 15, 2025) and still count for the 2024 tax year. The 2024 contribution limits are $4,150 for self-only coverage and $8,300 for family coverage, with an additional $1,000 for those 55 and older.
HSA contributions are deductible above the line, meaning they reduce your adjusted gross income whether or not you itemize.
8. Evaluate a Roth Conversion
A Roth conversion — moving funds from a traditional IRA or 401(k) into a Roth account — generates taxable income in the year of conversion, which may seem counterintuitive as a tax strategy. However, if your income is lower than usual in 2024, or if you have losses that can offset the conversion amount, moving funds now at a lower rate protects future growth from taxation entirely.
Conversions can technically be executed up to December 31 for the 2024 tax year, but given the administrative time required by custodians, initiating the process in early December is advisable.
9. Review Business Entity Structure for 2025
December is an appropriate time to evaluate whether your current business structure — sole proprietor, LLC, S-corporation — remains optimal given your income level. An S-corp election for 2025, for example, must generally be filed within the first two and a half months of the new tax year to be effective for that year. Beginning the analysis now gives you time to consult with a tax advisor and act promptly if a change is warranted.
A Quick-Reference Timeline
| Action | Deadline |
|---|---|
| Max out 401(k) employee deferrals | Last paycheck of 2024 |
| Harvest investment losses | December 31, 2024 |
| Purchase and place business equipment in service | December 31, 2024 |
| Make charitable contributions (direct or to DAF) | December 31, 2024 |
| Prepay deductible expenses (mortgage interest, etc.) | December 31, 2024 |
| Execute Roth conversion | December 31, 2024 |
| Fund HSA for 2024 | April 15, 2025 |
| SEP-IRA contributions for 2024 | April 15, 2025 (or extended deadline) |
| File S-corp election for 2025 | March 15, 2025 |
The Underlying Principle
Every strategy on this list operates on the same foundational logic: taxes are a function of timing and structure, not just income. The tax code does not reward passivity, but it consistently rewards deliberate, well-documented decisions made within the windows it provides.
December is the final window of the tax year. The moves you make in these remaining weeks will appear on a return filed months from now — but the decisions themselves expire on December 31. Act accordingly.
