October 15 – Partnerships.  File a 2005 calendar year return (Form 1065).  This due date applies only if you were given an additional 6-month extension.  Provide each partner with a copy of Schedule K–1 (Form 1065) or a substitute Schedule K–1.

October 15 – Electing large partnerships.  File a 2005 calendar year return (Form 1065–B).  This due date applies only if you were given an additional 6-month extension.  See March 15 for the due date for furnishing Schedules K–1 or substitute Schedules K–1 to the partners.

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.