September 15 – Corporations.  File a 2005 calendar year income tax return (Form 1120 or 1120–A) and pay any tax, interest, and penalties due.  This due date applies only if you timely requested an automatic 6-month extension.  Otherwise, see March 15.

September 15 – S corporations.  File a 2005 calendar year income tax return (Form 1120S) and pay any tax due.  This due date applies only if you timely requested an automatic 6-month extension.  Otherwise, see March 15.  Provide each shareholder with a copy of Schedule K–1 (Form 1120S) or a substitute Schedule K–1.

September 15 – Corporations.  Deposit the third installment of estimated income tax for 2006.  A worksheet, Form 1120–W, is available to help you estimate your tax for the year.

Note.  The 3 months that make up each quarter of a fiscal year may be different from those of each calendar quarter, depending on when the fiscal year begins.

Partnerships.  Form 1065.  This form is due on the 15th day of the 4th month after the end of the partnership's tax year.  Provide each partner with a copy of Schedule K–1 (Form 1065) or a substitute Schedule K–1.

Partnerships.  Form 1065–B (electing large partnerships).  This form is due on the 15th day of the 4th month after the end of the partnership's tax year.  Provide each partner with a copy of Schedule K–1 (Form 1065) or a substitute Schedule K–1.

Corporations and S Corporations.  Form 1120 and Form 1120S (or Form 7004).  These forms are due on the 15th day of the 3rd month after the end of the corporation's tax year.  S Corporations must provide each shareholder with a copy of Schedule K–1 (Form 1120S) or a substitute Schedule K–1.

Estimated tax payments.  Payments are due on the 15th day of the 4th, 6th, 9th, and 12th months of the corporation's tax year.

Form 2553.  This form is used to choose S corporation treatment.  It is due by the 15th day of the 3rd month of the first tax year to which the choice will apply or at any time during the preceding tax year.