Key takeaways
- Calendar-year Form 1120-S returns are due March 15, moving to the next day that is not a weekend or legal holiday when March 15 falls on one.
- Tax year 2025: original date March 16, 2026, extended date September 15, 2026. Tax year 2026: March 15, 2027 and September 15, 2027, derived from the rule until the IRS publishes them.
- Form 7004 moves the filing date only. Tax and any estimated tax deposit are due on the original date.
- A fiscal-year S corporation counts three months from its year-end, then uses the 15th.
- State due dates and extension forms differ, so the federal date cannot be copied into a state column.
The general rule
The instructions for Form 1120-S (2025 revision) state that an S corporation generally must file by the 15th day of the 3rd month after the end of its tax year. For a calendar-year corporation that is March 15. The same instructions say that if the due date falls on a Saturday, Sunday or legal holiday, the corporation can file on the next day that is not one of those days.
For tax year 2025, the IRS first-quarter tax calendar lists the Form 1120S entry on March 16, 2026. March 15, 2026 was a Sunday, so the due date moved to Monday, March 16.
2025 and 2026 dates
The tax year 2026 dates below are computed from the rule, not read from a published calendar entry. Confirm them against the 2026 instructions for Form 1120-S and the 2027 edition of Publication 509 when the IRS posts them, before anything goes on a client calendar.
Tax year 2025, filed in 2026. Original due date: March 16, 2026 (Monday), with any tax due paid that day. Extended due date: September 15, 2026 (Tuesday), for a corporation that filed Form 7004 by March 16, 2026. The third-quarter calendar lists the same date for timely 6-month extensions.
Tax year 2026, filed in 2027. Original due date: March 15, 2027 (Monday), no weekend shift. Extended due date: September 15, 2027 (Wednesday). Both come from the 15th-day-of-the-3rd-month rule and the 6-month Form 7004 extension.
Key the calendar by tax year, not filing year, so the tax year 2025 return (filed in 2026) stays in its own row.
Three dates for every return
For each return, set three dates: the original filing date, the extended filing date and the payment date. For a calendar-year tax year 2025 return those are March 16, 2026, September 15, 2026 and March 16, 2026.
The first-quarter calendar entry for S Corp says to file Form 7004 and deposit estimated tax for the automatic 6-month extension. Decision rule: if the corporation expects tax due and will file Form 7004, the payment and any estimated tax deposit go out on the original date. The September date moves only the filing deadline.
Fiscal-year S corporations
A fiscal-year S corporation does not file on March 15. Publication 509 (for use in 2026) states that Form 1120-S for a fiscal-year taxpayer is due on the 15th day of the 3rd month after the end of the corporation's tax year. Count three months forward from the year-end, then use the 15th.
Worked examples for tax years ending in 2026. A June 30 year-end is due September 15, 2026 and extended to March 15, 2027. A September 30 year-end is due December 15, 2026 and extended to June 15, 2027. A November 30 year-end is due February 16, 2027 and extended to August 16, 2027. The 15th falls on Washington's Birthday (February 15, 2027, a legal holiday) in the first case and on a Sunday (August 15, 2027) in the second, so both dates shift to the next business day.
Decision rule: if the client's year-end is not in December, do not use March 15 as the anchor. Take the year-end from the prior return and count three months before the template is applied.
Weekends, holidays and the internal review date
When a due date lands on a weekend, the federal filing date moves to the next day that is not a Saturday, Sunday or legal holiday.
The filing date moving does not move the review window. Decision rule: when the due date is a weekend, set the partner review and client signature deadline on the business day before. For tax year 2025 that is Friday, March 13, 2026, with the filing scheduled for Monday, March 16.
The extension and what it does not move
Publication 509 (for use in 2026) says Form 7004 is used to request an automatic 6-month extension of time to file Form 1120-S. The Instructions for Form 1120-S (2025 revision) say the corporation generally must file Form 7004 by the regular due date of the return.
Form 7004 extends the time to file, not the time to pay. The Instructions for Form 7004 (revised 12/2025) state that Form 7004 does not extend the time to pay any tax due.
If no extension was filed by the original date, the return is late. File as soon as possible and calculate the penalty in the next section. The instructions read for this guide do not describe a reasonable-cause relief path, so do not promise the client relief until the reviewing CPA has checked the current instructions.
Late-filing penalty for 2025 returns
For a return with no tax due, the Instructions for Form 1120-S (2025) set the late-filing penalty at $255 for each month or part of a month, up to 12 months, multiplied by the number of shareholders. That figure is from the 2025 revision, so check the instructions for the year the return is filed before quoting it to a client.
The shareholder count is every person who was a shareholder during any part of the tax year. A corporation with 3 shareholders at year-end that had a fourth shareholder who sold in May counts 4. Worked example: 4 shareholders times $255 times 2 months late is $2,040.
The minimum penalty applies to a return required to be filed in 2026 that is more than 60 days late: the smaller of the tax due or $525. For a tax year 2025 return that means a filing after May 15, 2026.
State due dates often differ
The New York Form CT-3-S instructions (2025 Form CT-3-S-I) say that if you are reporting for the calendar year, your return is due on or before March 15.
New York's extension is Form CT-5.4, Request for Six-Month Extension to File New York S Corporation Franchise Tax Return. The 2025 instructions say to pay the properly estimated franchise tax on or before the original due date of the return.
The sources here do not settle whether a federal Form 7004 covers a state return, so do not assume it does. Decision rule: each state where the corporation files gets its own column, with its own original date, extension form and payment date.
Using Usual for the calendar
Usual's recurring request generator lets you set a default due day for a request, aligns the schedule to each client's fiscal year, and lets you override the schedule for an individual client. Set the default to the original date for calendar-year clients, and check each fiscal-year client's generated dates against the year-end rule above rather than trusting a firm-wide March 15.
Failure modes and fixes
Failure: the tax year 2025 return is calendared for March 15, 2026. Fix: March 15, 2026 was a Sunday, so the federal due date is Monday, March 16, 2026. Change the filing entry to March 16 and move the review deadline to Friday, March 13.
Failure: a fiscal-year client is calendared on March 15 because the firm template is calendar-year. Fix: take the year-end, count three months and use the 15th. A June 30 year-end is due September 15, not March 15.
Failure: the extended September date is treated as the payment date. Fix: the payment and any estimated tax deposit go out on the original date with Form 7004. Only the filing date moves to September.
Failure: the state extension is assumed from the federal Form 7004. Fix: check the state's own extension form and deadline, and calendar it as a separate row.