For anyone with self-employment income or a small payroll, October 1, 2026 is not a soft target. It is the statutory last day to put two retirement plans in place for this tax year — and once it passes, nothing recovers it. Three other accounts stay open well into 2027, which is exactly why the deadline gets missed: savers assume the whole category runs on April 15.
What Closes Tomorrow
The SIMPLE IRA. The IRS is unusually literal here: an employer may set up a SIMPLE IRA plan "effective on any date from January 1 through October 1 of a year, provided you did not previously maintain a SIMPLE IRA plan." There is one narrow exception — a business that came into existence after October 1 may establish one as soon as administratively feasible. Employee deferrals cap at $17,000 for 2026, and the plan is limited to employers with roughly 100 or fewer employees.
A new safe harbor 401(k). There is no single code section printing "October 1" on a calendar; the date is arithmetic. A new calendar-year safe harbor plan's first plan year must run at least three months so every participant gets a genuine opportunity to defer. Count backward from December 31 and the effective date can be no later than October 1 — and the plan has to be functioning by the first payroll on or after that date, not merely signed.
In both cases, "established" means employees can actually make salary deferral elections in time to apply them to October-through-December compensation. A signature with no payroll mechanics behind it does not satisfy the rule.
What Is Still Open
SEP-IRA. The most forgiving deadline in the code. Per the IRS, "you can set up a SEP plan for a year as late as the due date (including extensions) of your business's income tax return for that year," and contributions follow the same clock. A sole proprietor on extension has until October 15, 2027, to establish and fund a 2026 SEP.
Solo 401(k). Corporations and partnerships must sign plan documents by December 31, 2026. Sole proprietors and single-member LLCs with no employees get more room: SECURE 2.0 Section 317 lets them adopt a first-year plan and make retroactive employee deferrals up to the tax return filing date — but explicitly "without extensions." That is April 15, 2027, and filing a 4868 does not move it.
Traditional and Roth IRAs. April 15, 2027, for 2026 contributions. Extensions never apply.
The Exclusivity Trap
One rule quietly outranks all of the above: a SIMPLE IRA cannot be maintained in any year the employer also maintains another retirement plan that provides a benefit. That forecloses the common instinct to "just add a SIMPLE now and sort out a 401(k) later." It also means a mid-year switch into a SIMPLE is generally off the table.
Practical Takeaways
- Separate the signature from the plumbing. If you are racing tomorrow's deadline, confirm your payroll provider can process a deferral election this week. A plan document alone does not make the date.
- Missing October 1 is not the end of 2026. If you have self-employment income and no staff, a solo 401(k) still captures both employee deferrals and employer contributions for this year.
- Watch the "without extensions" asymmetry. The Section 317 solo 401(k) deferral deadline and the SEP deadline look similar and behave differently. Extensions help one and not the other.
- Existing SIMPLE sponsors: November 1 is next. The annual summary description and election notice must reach employees roughly 60 days before the next plan year begins.
Sources: IRS, "SIMPLE IRA Plan" (Plan Sponsor); IRS, "Retirement Plans FAQs Regarding SEPs"; Ascensus, "Things to Consider as SIMPLE IRA Deadline Looms" (September 14, 2026); Employee Fiduciary, "401(k) Adoption Deadlines: SECURE Act Changes"; Schneider Downs, "SECURE 2.0 Act Section 317 Summary"; SECURE 2.0 Act, Section 317.