September 15 is the due date for the third quarterly installment of the 2010 estimated tax for calendar year corporations.  For fiscal year corporations, estimated payments fall due on the fifteenth day of the fourth, sixth, ninth, and twelfth months of the taxable year.

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.