Missing a tax deadline is one of those mistakes that feels small until the penalty notice shows up. Late filing, late payment, underpaid estimates — the IRS and California's Franchise Tax Board both charge for these, and the charges are not trivial. The simplest way to avoid them is to know what's due and when.

We put this calendar together for our clients, but it applies to anyone filing in California. Some of these dates are obvious. A few of them aren't.

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January 2026 — It Starts Before You Think

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January 15, 2026: Fourth quarter estimated tax payment for the 2025 tax year is due — both federal (IRS) and state (FTB). If you're a business owner or self-employed individual who has been making quarterly estimates, this is the last installment for the prior year. Miss this one and you'll owe an underpayment penalty on top of whatever balance is due when you file.

There's an exception worth knowing. If you file your 2025 return and pay the full balance by January 31, you can skip the January 15 estimated payment without penalty. That's a tight window, but for people whose books are clean and who have all their documents early, it's doable.

January 31, 2026: Employers must furnish W-2s to employees and file them with the Social Security Administration. 1099-NEC forms for independent contractors are also due to recipients and the IRS by this date. If you hire subcontractors and haven't sent their 1099s yet, this is not optional — and the penalties for late filing are per form.

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March 2026 — Business Returns Come First

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March 16, 2026: This is the filing deadline for partnerships (Form 1065) and S-Corporations (Form 1120-S). These entities don't pay income tax themselves, but they issue Schedule K-1s to their owners, and those K-1s are needed to complete individual returns.

This is where things cascade. If your partnership or S-Corp return is late, you're not just facing a penalty on the business return — you're potentially delaying your personal return and every other owner's personal return. The penalty for late filing of a partnership return is $220 per partner per month, up to 12 months. For a four-partner firm that files three months late, that's $2,640 in penalties for a return that might not even owe any tax.

We file extensions for most business returns that aren't ready by March 16, but the extension needs to be filed on time — not retroactively.

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April 2026 — The Big One

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April 15, 2026: Individual income tax returns (Form 1040) are due. This is also the deadline for C-Corporation returns (Form 1120) if you're on a calendar year. And it's the due date for the first quarter 2026 estimated tax payment.

Three separate obligations, one date. We see people focus on their return and forget about the estimated payment for the current year. Don't be that person.

If you need more time to file, Form 4868 gives you an automatic six-month extension to October 15. But here's the part people misunderstand every single year: an extension to file is not an extension to pay. If you owe money, it's due April 15 regardless of whether you've extended. Interest starts accruing on any unpaid balance the next day. The late payment penalty kicks in too — typically 0.5% of the unpaid tax per month.

Our recommendation: if you're extending, estimate what you owe and send a payment with the extension. Overpaying slightly is better than underpaying and getting hit with interest.

April 15 is also the last day to make a Traditional IRA or Roth IRA contribution for the 2025 tax year. If you haven't maxed out your contribution, this is your last shot. A Traditional IRA contribution can reduce your taxable income on the return you're about to file — it's one of the few retroactive tax planning moves available.

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May Through September — The Dates People Forget

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May 15, 2026: Tax-exempt organizations must file Form 990 by this date. If you're involved with a nonprofit, make sure this is on someone's calendar.

June 15, 2026: Second quarter estimated tax payment due. Also the filing deadline for U.S. citizens living abroad — they get an automatic two-month extension, but interest on any unpaid balance still runs from April 15.

September 15, 2026: Third quarter estimated payment due. This is also the extended deadline for partnership and S-Corporation returns that filed for an extension back in March. And here's a detail that catches people: this is also the deadline for SEP-IRA contributions if you filed an extension for your business return. If you're self-employed and extended, you have until this date to set up and fund a SEP for the prior tax year.

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October 2026 — Extended Returns Come Due

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October 15, 2026: Extended individual and C-Corporation returns are due. No further extensions available. If you haven't filed by this date, you're officially late, and the failure-to-file penalty — which is significantly steeper than the failure-to-pay penalty — begins to accrue at 5% of the unpaid tax per month, up to 25%.

We start reaching out to extended clients in August to make sure we're not scrambling in October. If you've ever been in the position of realizing on October 14 that you're missing a document, you know why.

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California-Specific Dates

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California generally follows federal due dates for individual and business returns, but there are some differences worth noting.

The FTB requires its own estimated tax payments on the same quarterly schedule, but California's payment percentages are different. California requires 30% with the first payment, 40% with the second, 0% with the third, and 30% with the fourth. Compare that to the federal schedule, which requires 25% each quarter. This means your California estimate is front-loaded — a larger share is due in April and June than most people expect.

If you're affected by a federally declared disaster in California, the IRS and FTB may extend deadlines for affected areas. This has happened several times in recent years due to wildfires and flooding. We monitor these announcements and notify affected clients, but it's worth keeping an eye on if you're in a fire-prone area of the county.

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One Final Thought

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A calendar is only useful if someone's actually watching it. The clients who never have deadline problems aren't the ones with perfect memories — they're the ones who have a system. Whether that's a tax professional tracking dates for you, automated reminders, or a spreadsheet you actually check, the tool matters less than the habit.

If you'd rather have someone else worry about this, that's what we do. San Diego Tax Associates tracks every deadline for every client — so you don't have to.

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