Fall is one of the busiest seasons on the tax calendar. Whether you filed an extension back in April or you're a business owner making quarterly estimated payments, the next few months are packed with important dates. Missing them can mean penalties, interest, and unnecessary stress — none of which you need.

Here's what to put on your calendar right now.

September 15 — Q3 Estimated Tax Payment Due

If you're self-employed, a freelancer, a landlord, or a business owner who doesn't have taxes withheld from a paycheck, your third-quarter estimated tax payment was due September 15.

If you missed it, don't wait. Pay as soon as possible to minimize the underpayment penalty. The IRS calculates that penalty daily, so every day counts.

Who this applies to:

• Sole proprietors and single-member LLCs
• Partners and S-corporation shareholders
• Rental property owners with significant income
• Anyone who expects to owe $1,000 or more at filing

Not sure how much to pay? A good rule of thumb is to pay at least 100% of last year's tax liability (or 110% if your adjusted gross income exceeded $150,000). We can help you calculate the right amount.

October 15 — Extended Individual Returns Due

If you filed for an extension in April, October 15 is your hard deadline for submitting your 2025 federal and California individual income tax returns. There are no further extensions available after this date.

This is one of the most commonly missed deadlines we see. Life gets busy, and it's easy to assume you have more time than you do. You don't.

What to have ready:

• All W-2s and 1099s
• Investment and brokerage statements
• Rental income and expense records
• Business income and expense summaries
• Last year's return for reference
• Health insurance forms (1095-A, B, or C)

If you haven't started gathering documents yet, now is the time. The closer we get to October 15, the harder it becomes to get an appointment and ensure accuracy.

October 15 — Extended Business Returns (S-Corps and Partnerships)

S-corporations and partnerships that filed for an extension also face an October 15 deadline for their 2025 returns (Form 1120-S and Form 1065, respectively).

These returns flow through to individual shareholders and partners, so delays here can create a domino effect on personal returns. If your business return isn't filed, your personal return can't be finalized either.

November — Year-End Tax Planning Window Opens

Once the October filing rush is behind us, November is the ideal time to start thinking about year-end tax planning strategies before December 31 closes the door on 2026 deductions.

Some moves to consider before year-end:

• Maximize retirement contributions — Traditional IRA, SEP-IRA, and Solo 401(k) contributions can reduce your taxable income significantly.
• Harvest investment losses — If you have losing positions in a taxable brokerage account, selling before December 31 can offset capital gains.
• Accelerate deductible expenses — If you're a business owner, consider purchasing equipment or prepaying deductible expenses before year-end.
• Review your withholding — If you've had a major life change (new job, marriage, home purchase, new dependent), now is a good time to adjust your W-4 to avoid a surprise bill in April.

January 15, 2027 — Q4 Estimated Tax Payment Due

Mark this one now. The fourth and final estimated tax payment for 2026 is due January 15, 2027. This covers income earned October through December.

Alternatively, if you file your full 2026 return and pay any balance due by January 31, 2027, you can skip the January 15 estimated payment. This is a useful option if you're ready to file early.

Don't Navigate This Alone

Tax deadlines are unforgiving, and the penalties for missing them add up quickly. At Automated Business Service, we work with individuals and small businesses throughout Hesperia and the High Desert year-round — not just during tax season.

If you have an extended return due October 15, need help calculating your Q3 estimated payment, or want to get ahead of year-end planning, give us a call at (760) 947-8060 or stop by our office at 16501 Walnut St., Ste. 1, Hesperia. We're here to make sure you never miss a deadline that costs you money.