Missing a tax deadline doesn't just mean filing late. It means penalties that start immediately and compound monthly, plus interest on any unpaid tax. For some deadlines — particularly payroll tax deposits — the penalties are steep enough to meaningfully damage a business's cash position.
Here's a complete calendar of business tax deadlines with context for why each one matters.
When a deadline falls on a weekend or federal holiday, it moves to the next business day.
January Deadlines
January 15
— Q4 estimated tax payment due for individuals and sole proprietors. This is the final quarterly payment for the prior tax year. If you haven't made sufficient payments throughout the year, this payment reduces your underpayment penalty.
January 31
— W-2s must be provided to employees and filed with the Social Security Administration. Send 1099-NEC forms to contractors you paid $600 or more for work in 2025. For payments made in 2026 and later, the threshold rises to $2,000. Penalties apply per late form and increase the longer you wait to file.
March Deadlines
March 15
— Partnership returns (Form 1065) and S-Corp returns (Form 1120-S) are due. This is also the deadline to file Form 2553 to elect S-Corp status for the current tax year. Missing the S-Corp election deadline means waiting until next year — and paying self-employment tax at the higher rate for another full year.
A six-month extension is available by filing Form 7004. The extension moves the filing deadline to September 15 but does not extend the time to pay any tax due.
Missed deadlines bring penalties that add up fast. Let us build a filing calendar around your federal and Washington obligations.
April Deadlines
April 15
— Individual tax returns (Form 1040) are due, including Schedule C for sole proprietors. C-Corp returns (Form 1120) are also due. This is also the Q1 estimated tax payment due date for the current year.
Three significant deadlines converging on the same date is why April is the most consequential month in the tax calendar for small business owners.
Individual and C-Corp filers can extend to October 15 by filing Form 4868 or Form 7004 by April 15. Extensions are automatic — no explanation required. But they extend the filing deadline only, not the payment deadline. Tax owed is still due April 15.
June Deadlines
June 15
— Q2 estimated tax payment for the current year. Note that Q2 covers only April and May — two months, not three. The irregular schedule catches some people off guard.
September Deadlines
September 15
— Q3 estimated tax payment for the current year. Also the deadline for extended partnership and S-Corp returns.
October Deadlines
October 15
— Extended individual and C-Corp returns are due. This is the final deadline — no further extensions are available for these return types.
December Deadlines
December 31
— The last day for several year-end strategies.
Solo 401k employee contributions and plan establishment must be completed by December 31.
Equipment purchases must be placed in service by December 31 to qualify for Section 179 expensing or bonus depreciation in the current year.
Business expenses must be paid by December 31 to be deductible in the current year under cash basis accounting.
Charitable contributions must be completed by December 31 to qualify for a current-year deduction.
Payroll Tax Deposit Deadlines
Separate from annual filing deadlines, payroll tax deposits have their own schedule.
Most small businesses are monthly depositors — federal payroll taxes for each payroll period are due by the 15th of the following month. Semi-weekly depositors have more frequent requirements.
The Trust Fund Recovery Penalty for late payroll tax deposits can be assessed personally against any officer or owner with responsibility for remitting payroll taxes — meaning it bypasses the business entity entirely and attaches to you personally. The penalty equals 100 percent of the unpaid trust fund taxes.
Payroll tax deposits need to happen on schedule, every pay period, without exception.
Washington State Deadlines
Washington State B&O tax returns are due on a quarterly or annual schedule depending on your revenue. Quarterly filers are due on the last day of the month following each quarter — April 30, July 31, October 31, and January 31.
Seattle, Bellevue, Tacoma, and other cities with their own B&O tax have separate filing schedules.
Washington Paid Family and Medical Leave premiums are remitted quarterly along with state unemployment insurance, due on the last day of the month following each quarter.
Building Your Own Calendar
The most reliable system is a dedicated calendar with reminders set two weeks before each due date. Two weeks gives you enough time to gather what's needed, resolve questions, and make payments or file returns without rushing.
Handling this yourself? A structured calendar is the simplest and most reliable system available. For our clients, managing that calendar is part of the ongoing relationship.