Tax Season Deadline Calendar for Accounting Firms

A tax season deadline calendar for accounting firms lists the federal due dates from January to October: information returns, partnership and S corporation returns, individual and C corporation returns, trust returns, estimated payments and extended returns. Beside each IRS date sits the firm's earlier finish date for preparation, review and client sign-off, so work ends before the deadline does.

IRS due dates on a tax season deadline calendar for accounting firms

The IRS due dates on a firm calendar come from one source: IRS Publication 509 (2026), Tax Calendars. It prints the 2026 dates for tax year 2025 returns, already moved off weekends and holidays. A firm needs six groups of dates: information returns, Forms 1065 and 1120-S, Form 1040, Form 1120, Form 1041 and extended returns.

Every date in this section is for tax year 2025, filed in calendar year 2026, because that is the year Publication 509 (2026) covers. The next season's dates appear when the IRS publishes the following edition. Until then, the general rules Publication 509 states let a firm see which month each form lands in: Form 1065 and Form 1120-S are due on the 15th day of the 3rd month after the tax year ends, and Form 1040 and Form 1120 on the 15th day of the 4th month.

Information returns in January, February and March

Information returns are the W-2s and 1099s a firm's business clients issue. Publication 509 (2026) lists these tax year 2025 dates:

• February 2, 2026. Employers give employees their 2025 Forms W-2 and file Form W-3 with Copy A of every W-2. Payers file Form 1099-NEC for nonemployee compensation paid in 2025. Most other 1099s are due to recipients.
• February 17, 2026. Forms 1099-B, 1099-S and certain Form 1099-MISC amounts are due to recipients.
• March 2, 2026. Paper filing of information returns such as Forms 1098 and 1099 (other than Form 1099-NEC) with the IRS.
• March 31, 2026. The same forms, if filed electronically.

Payroll clients also have Form 941 for the fourth quarter of 2025 and Form 940 for 2025 due February 2, 2026, or by February 10 if the tax was deposited timely, properly and in full. When those 1099s reach a firm's individual clients is a separate question, covered in which 1099s are due to recipients by January 31.

Business and individual returns

• March 16, 2026. Calendar-year Form 1065 for 2025 and Form 1120-S for 2025, with each partner's or shareholder's Schedule K-1. Form 7004 requests an automatic 6-month extension. Publication 509 also lists this date for filing Form 2553 to elect S corporation treatment beginning with calendar year 2026, and says a late Form 2553 moves the start to calendar year 2027.
• April 15, 2026. Form 1040 or 1040-SR for 2025, with Form 4868 for an automatic 6-month extension. Calendar-year Form 1120 for 2025, with Form 7004 for an extension.
• April 15, 2026. Form 1041 and Schedules K-1 for calendar-year estates and trusts, according to the Instructions for Form 1041 (2025). Publication 509 does not cover trusts, so this date comes from the form instructions.

Extended returns

• September 15, 2026. Extended 2025 Forms 1065 and 1120-S, for returns with a timely Form 7004.
• October 15, 2026. Extended 2025 Form 1040 and Form 1120.

The Form 1041 extension is described differently. The Instructions for Form 1041 (2025) say Form 7004 gives an automatic 5½-month extension and do not print a calendar date for it. Confirm the date in the current instructions before putting an extended trust return on the calendar.

Which internal firm deadlines sit before each IRS due date?

Internal firm deadlines are the dates a firm sets for its own stages, earlier than the IRS date. A workable set is three: preparation finished, review finished, and client sign-off. Each is a fixed number of business days before the due date, sized from how long that stage took last season, plus a margin for the busiest weeks.

The document cutoff comes before all three. It is the date a client's file must be complete for the firm to commit to the original due date, and the arithmetic for setting it lives in setting extension cutoff dates for clients. The date the firm gives clients for sending documents is a separate, earlier date, covered in setting a document deadline for tax clients. This section starts where those stop, once the file is complete. What each stage must deliver before a return leaves it is set in the stages of a tax season workflow; the dates below only say when.

Client sign-off

Sign-off is the last internal date, and the one easiest to leave too close. Some clients answer the same day. Others take a week, especially business owners in their own busy months. A sign-off date at least five business days before the due date gives room for one round of questions.

Review finished

The review finish date sets when the reviewer's work must be done. Set it a few business days before sign-off. If last season's review notes took an average of two days to clear, and returns waited three days in the review queue, the review finish date sits at least five business days before sign-off.

Preparation finished

The preparation finish date sits before the review date by the number of days returns waited in the review queue last season. For returns with K-1s, rentals or several states, add more time, because those returns tend to come back with more notes.

Where the internal dates go

Put the internal dates on the same calendar as the IRS dates, one line per return type, not per client. A preparer looking at March should see "1065 preparation finished" and "1065 review finished" above "Form 1065 due." When the internal dates live only in a manager's head, staff work to the IRS date, and the margin disappears.

Adding estimated tax payment dates to the firm calendar

Estimated tax payment dates belong on a firm calendar because they fall inside the season and right after it. Individuals pay quarterly using Form 1040-ES, and corporations deposit in four installments. Each date needs a named person who tells the affected clients in advance.

Form 1040-ES (2026) prints the payment due dates for tax year 2026 estimated tax:

• 1st payment: April 15, 2026
• 2nd payment: June 15, 2026
• 3rd payment: September 15, 2026
• 4th payment: January 15, 2027

The form notes that the January 15, 2027 payment is not required if the 2026 return is filed by February 1, 2027 and the entire balance is paid with it. The fourth payment for tax year 2025 was due January 15, 2026, which Publication 509 (2026) lists as the final installment date for 2025 estimated tax.

For corporations, Publication 509 (2026) says estimated tax payments are due on the 15th day of the 4th, 6th, 9th and 12th months of the corporation's tax year. For a calendar-year corporation in 2026 it lists the installments on April 15, June 15, September 15 and December 15.

Who tells clients

The calendar should name an owner for each estimated payment date, usually the preparer assigned to the client. The owner's job is to send each affected client the amount and the date before it arrives, not to decide the amount. A list of clients who make estimated payments, kept with last year's return, makes this a short task. Without one, the April payment gets buried under the April return, and the June payment is forgotten because the season feels over.

The April 15 overlap deserves its own line on the calendar. On the same day a client files a 2025 return or extension, the first 2026 payment is due. A note on the April line reminds preparers to mention the first installment in the extension conversation, which the extension post covers in how to explain an extension to a client.

How do weekends, holidays and disaster relief move a tax deadline?

A tax deadline moves for three reasons: weekends and legal holidays, IRS disaster postponements, and state rules. Publication 509 says an act due on a Saturday, Sunday or legal holiday is timely if done on the next day that is not one of those. Disaster relief moves dates only for taxpayers in the areas the IRS names.

The next-business-day rule

Publication 509 (2026) states that "legal holiday" means a legal holiday in the District of Columbia, and that its calendars already make the adjustment for Saturdays, Sundays and legal holidays. That is why it prints February 2, 2026 for forms due the last day of the first month after the year ends, and March 16, 2026 for returns due on the 15th day of the 3rd month. A firm that copies dates from Publication 509 does not need to adjust them again. A firm that writes dates from the general rule does.

The 2026 legal holidays Publication 509 lists include April 16, District of Columbia Emancipation Day, the day after the April 15 deadline. The IRS page When to file gives the same rule for individuals: if day 15 falls on a Saturday, Sunday or legal holiday, the due date moves to the next business day.

Statewide holidays

A statewide legal holiday can also move a federal date. Publication 509 (2026) says it delays a filing due date only if the IRS office where the return must be filed is in that state, and for individuals also for residents of that state. It does not delay a federal tax deposit.

Disaster postponements

The IRS postpones deadlines for taxpayers in areas hit by a declared disaster. Its page Tax relief in disaster situations lists each announcement by date and points to the current list of eligible localities. One listed example: relief for taxpayers affected by severe storms and tornadoes in Wisconsin, with various deadlines postponed to February 1, 2027. Relief applies by locality, so a firm with clients in several counties checks each client's address against the announcement and marks the postponed date on that client's line only.

State dates that differ

Publication 509 covers federal dates only. State income tax returns follow each state's own rules, and a state date may not match the federal one. Add each state's dates from that state's revenue department as a separate line, so nobody assumes the federal date applies.

Deadline calendar mistakes: tracking only April 15

The deadline calendar mistake with the widest effect is building the calendar around Form 1040 and April 15. A firm with business clients has dates in February, March, June, September and October that matter as much. Missing a March business date means late K-1s for every owner of that business, which then delays their individual returns.

March dates firms miss

March holds two kinds of dates. The March 16, 2026 date for tax year 2025 Forms 1065 and 1120-S comes a month before the individual deadline, and an individual return that needs a K-1 cannot be finished until the K-1 exists. The March 2 and March 31 information return filing dates catch firms that file 1099s for clients and treat February as the end of that work.

September dates firms miss

September 15, 2026 is the extended due date for tax year 2025 partnership and S corporation returns, and the third 2026 estimated payment date. Firms that extend many business returns in March find September crowded, and the extension post covers spreading extended returns from May to October.

October dates firms miss

October 15, 2026 is the extended due date for tax year 2025 Forms 1040 and 1120. It is often treated as the end of the season, but every extended return the firm filed in April comes due here at once.

Other mistakes

• One calendar per person. If each preparer keeps a private calendar, the firm has no single view. Keep one shared calendar with the IRS date and internal dates on the same line.
• No tax year on the line. "1065 due March 16" is ambiguous a year later. Write "Form 1065, tax year 2025, due March 16, 2026."
• Dates copied from last year. The day of the month shifts when a due date lands on a weekend or holiday. Copy each year's dates from the current Publication 509, not from last year's calendar.

A month-by-month firm deadline calendar

The table below is an example for a fictional firm, Brookfield Example CPAs, with calendar-year clients. The IRS due dates are for tax year 2025 and come from Publication 509 (2026), the Instructions for Form 1041 (2025) and Form 1040-ES (2026). The internal finish dates are the example firm's own targets, not IRS dates.

The internal dates only hold if the firm knows, on each preparation date, which files are complete. Schema CPA compares every document a client sends with what came in for last year's return and lists what is still missing, so the file that is not ready shows up before its preparation date rather than on it. The workflow section of the homepage shows how. Once the calendar is set, preparer time for client meetings has to fit around it, and scheduling client appointments in tax season covers how to block those hours.