Illinois business filing dates, 2027

“In general, Form IL-1065 is due on or before the 15th day of the 4th month following the close of the taxable year. We grant you an automatic six-month extension of time to file your partnership tax return.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“a date other than June 30, Form IL-1120 is due on or before the 15th day of the 4th month following the close of the tax year.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“For tax years ending on or after December 31, 2021, if your tax year ends on: a date other than June 30, we grant you an automatic extension of time to file your annual return of seven months.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“If you are a cooperative, Form IL-1120 is due on the 15th day of the 9th month following the close of the tax year regardless of when your tax year ends.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“You must file Form IL-1120, Corporation Income and Replacement Tax Return, if you are a corporation that has net income or loss as defined under the Illinois Income Tax Act (IITA); or is qualified to do business in the state of Illinois and is required to file a federal income tax return (regardless of net income or loss).”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Your first estimated payment is due by April 15, 2026. You may either pay all of your estimated tax at that time or pay your estimated tax in four equal installments that are due on April 15, 2026; June 15, 2026; September 15, 2026; and January 15, 2027. Note: If you file on a fiscal-year basis, please adjust all the due dates to correspond to your tax year.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Estimated payments are due on the 15th day of the 4th, 6th, 9th, and 12th months of the tax year. Note: If a due date falls on a weekend or holiday, the payment will be due on the next business day.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Corporations filing Form IL-1120, Corporate Income and Replacement Tax Return, who reasonably expect their income tax and replacement tax liability after subtracting Illinois withholding, pass-through withholding payments, and tax credits to be more than $400, must make quarterly estimated payments. Partnerships filing Form IL-1065, Partnership Replacement Tax Return, and subchapter S corporations filing Form IL-1120-ST, Small Business Corporation Replacement Tax Return, who elect to pay PTE tax and who reasonably expect their replacement tax and PTE tax liability after subtracting Illinois withholding, pass-through withholding payments, and tax credits to be more than $500, must make quarterly estimated payments.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“1. January, February, March | March 31, 2026 | April 30, 2026”verbatim · tax.illinois.gov · retrieved 2026-09-23

“2. April, May, June | June 30, 2026 | July 31, 2026”verbatim · tax.illinois.gov · retrieved 2026-09-23

“3. July, August, September | September 30, 2026 | November 2, 2026”verbatim · tax.illinois.gov · retrieved 2026-09-23

“4. October, November, December | December 31, 2026 | February 1, 2027”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Unlike the federal government, Illinois does not require an annual reconciliation return.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“You must file Form IL-941, Illinois Withholding Income Tax Return, if you paid amounts subject to Illinois withholding income tax”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Form IL-941 is the only form used to report Illinois income tax withholding with the exception of household employee withholding, which can be reported on Form IL-1040, Individual Income Tax Return.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Employers must file wage reports (Form UI-3/40) and pay contributions in the month after the close of each calendar quarter - that is, on or before April 30, July 31, October 31, and January 31.”verbatim · ides.illinois.gov · retrieved 2026-09-23

“Governmental organizations and some non-profits are also required to file wage reports, although they may have an option to reimburse IDES for benefits paid rather than making quarterly contributions.”verbatim · ides.illinois.gov · retrieved 2026-09-23

“You must file your Form ST-1 and pay any tax, E911 Surcharge, and ITAC Assessment due on or before the 20th day of the month following the end of the reporting period. Due dates that fall on Saturdays, Sundays, and State of Illinois holidays are moved to the next business day. For most taxpayers, the reporting period is over at the end of the month and the return is due on or before the 20th day of the month following the end of the reporting period. Some taxpayers qualify to file only four times a year (quarterly). These taxpayers’ returns are due on the 20th day of the month following the end of the calendar quarter. Still other taxpayers qualify to file only once a year (annually). These taxpayers’ returns are due on the 20th day of January following the end of the calendar year.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“You must file your Form ST-1 and pay any tax, E911 Surcharge, and ITAC Assessment due on or before the 20th day of the month following the end of the reporting period. Due dates that fall on Saturdays, Sundays, and State of Illinois holidays are moved to the next business day. For most taxpayers, the reporting period is over at the end of the month and the return is due on or before the 20th day of the month following the end of the reporting period. Some taxpayers qualify to file only four times a year (quarterly). These taxpayers’ returns are due on the 20th day of the month following the end of the calendar quarter. Still other taxpayers qualify to file only once a year (annually). These taxpayers’ returns are due on the 20th day of January following the end of the calendar year.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“You must file your Form ST-1 and pay any tax, E911 Surcharge, and ITAC Assessment due on or before the 20th day of the month following the end of the reporting period. Due dates that fall on Saturdays, Sundays, and State of Illinois holidays are moved to the next business day. For most taxpayers, the reporting period is over at the end of the month and the return is due on or before the 20th day of the month following the end of the reporting period. Some taxpayers qualify to file only four times a year (quarterly). These taxpayers’ returns are due on the 20th day of the month following the end of the calendar quarter. Still other taxpayers qualify to file only once a year (annually). These taxpayers’ returns are due on the 20th day of January following the end of the calendar year.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“Taxpayers whose average tax due on Form ST-1 is at least $20,000 a month must make four tax payments each month and must make them electronically. Payments are due on the 7th, 15th, 22nd, and last day of the month. IDOR will notify you if you meet this requirement.”verbatim · tax.illinois.gov · retrieved 2026-09-23

“*Note: If the due date falls on a weekend or holiday, the return is due on the next business day.”verbatim · tax.illinois.gov · retrieved 2026-09-23