September 15 is the last day a calendar year partnership can file Form 1065 without penalty if an extension has been granted, pursuant to Form 7004.

Before you mail any check to the Internal Revenue Service, be sure to complete the following: 

• Note the reason for the payment, the form number and the year of the return for which the check is being sent. For example, "balance due on Form 1040 for 2008."
• Include your social security number or your employer identification number if you operate as a business.
• Make all checks payable to the U.S. Treasury.