Tax season checklist for accounting firms: October to October, with the dates

By Jonathan Solomon, Founder of LucaLedger. Last updated September 2026. Dates are for the 2027 filing season (2026 returns), read from the IRS sources at the bottom.

The short answer A tax season runs well when the setup happens in October and December, not January: PTINs renewed, the security plan reviewed, engagement letters signed, organizers out, December books reconciled, and a document cutoff every client has read. After that the season is a calendar: February 1 for W-2s and 1099-NECs, March 15 for pass-throughs, April 15 for everyone else, and a post-season review before the extensions in September and October.

Tax season checklist (Word): the same list below as tick boxes, with the sources.

October to November

Renew every preparer's PTIN. Renewal opens in October, PTINs expire December 31, and the fee is $18.75.
Confirm the firm's EFIN and e-file application are current and list the right responsible officials.
Review and re-sign the Written Information Security Plan and have every staff member re-acknowledge it.
Move 1099 filing from FIRE to IRIS. FIRE takes its last filings at 3 p.m. Eastern on November 19, 2026.
Update engagement letter templates: fees, the document cutoff date, extension language, e-signature.
Set the document cutoff (two to three weeks before the deadline) and write down the extension policy.
Build the organizer for each client type and set the send date.

December

Send engagement letters for signature. No work starts on an unsigned engagement.
Send organizers and document request lists built from last year's return, one line per document.
Reconcile every business client's December so year-end books are final before 1099s go out.
Pull the 1099 vendor list and chase missing W-9s. For 2026 payments the 1099-NEC threshold is $2,000.
Check that every client's portal login works and their phone and email are current.
Social Security's Business Services Online opens for tax year 2026 W-2 uploads in December; check the date in the year's EFW2 specifications.

January

January 15, 2027: fourth-quarter 2026 estimated payments are due.
First organizer reminder two weeks before the cutoff, listing only what is still missing.
Confirm the IRS e-file opening date and send a test transmission.
February 1, 2027 (January 31 is a Sunday): W-2s to employees and the SSA, and 1099-NECs to contractors and the IRS.

February

Other 1099s to recipients by February 1; to the IRS by March 1 on paper or March 31 electronically.
Second and third reminders one week and one day before the cutoff.
Start each return the day its documents are complete. Do not batch them for March.
Every return gets a second review before it goes out for signature.

March

March 15, 2027: partnership (1065) and S corporation (1120-S) returns and K-1s, or Form 7004 extensions.
The cutoff passes: move late clients to extension and tell them what an extension does and does not do.
Estimate the tax for every extended client so a payment goes with the extension.

April

April 15, 2027: individual (1040), C corporation (1120) and trust (1041) returns; Form 4868 and 7004 extensions; first-quarter 2027 estimates.
Extension payments are due April 15. An extension moves the filing date, never the payment.
Send the invoice with the signed return and collect before filing where the engagement letter says so.

After the deadline

Post-season review: what took longest, which clients were late, what the engagement letter should say next year.
June 15 and September 15, 2027: second and third estimates.
September 15, 2027: extended 1065 and 1120-S returns. October 15, 2027: extended 1040 and 1120 returns.
Update the client list, the fee schedule and the WISP review date.

The eight steps every return goes through

The checklist above is the season. Each return inside it runs the same eight steps, and the list of what the firm does by hand is short when the platform does the rest.

Send the organizer. Pick the template, set the due date. It reaches the client's portal with a notification and a reminder schedule.
Receive documents. Each upload is read and routed to the right tile: W-2, 1099, K-1, statements. You review the uncertain matches.
Categorize transactions. Vendor history and your rules assign categories; you approve in batches and override the exceptions.
Prepare the return. The 1040 and every schedule populate from approved books; diagnostics run and carryovers are tracked.
Strategy review. Candidate strategies with the code section or ruling behind each, for the client conversation.
Review and sign-off. The diagnostics checklist, overridden lines highlighted, then lock the package. The review checklist is the published version of those rules.
Client signature. Package and engagement letter to the portal, tamper-evident signature trail, notification on signature.
Export and close. Signed return PDF for your e-file software, invoice drafted, workflow closed.

Frequently asked questions

When should an accounting firm start preparing for tax season?

October. PTIN renewal opens in October, the WISP review and engagement letter updates take a few weeks, and organizers need to reach clients in December if documents are to arrive in January. Firms that start in January are booked out by the end of it.

What is the document cutoff date and why does it matter?

The date after which a client's documents may push the return to an extension. Put it in the engagement letter, two to three weeks before the filing deadline, and repeat it in every reminder. It turns the last week of a season from a pile into a policy.

Which deadlines change in the 2027 season?

January 31, 2027 is a Sunday, so W-2s and 1099-NECs are due February 1. The 1099-NEC threshold for 2026 payments is $2,000, not $600. IRS e-filing of 1099s moves entirely to IRIS because FIRE stops on November 19, 2026. March 15, April 15, June 15, September 15 and October 15, 2027 all fall on weekdays.

Should we file extensions for every late client?

File extensions for every client whose documents are not complete by the cutoff, with a tax estimate and payment. A late-filed return with no extension carries a failure-to-file penalty; an extension with an underpayment carries only interest and, in some cases, a smaller penalty.

Sources

IRS, PTIN requirements for tax return preparers: renewal window, December 31 expiry, $18.75 fee.
2026 Instructions for Forms W-2 and W-3 and Publication 1099 (2026): February 1, 2027 and the 1099 dates and thresholds.
IRS news release IR-2026-99: FIRE retirement on November 19, 2026.
IRS Publication 509, Tax Calendars: return and estimated payment due dates; weekend and holiday rule.
State tax deadlines 2027 on this site for every state's dates and extension rules.

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Dates move to the next business day when they fall on a weekend or legal holiday. Confirm with the IRS and your state before relying on a date. This is general information, not tax advice.