The due date for corporations whose tax year ends on December 31st is March 15. If you are unable to file by then, most domestic corporations can apply for an automatic 6 month extention using IRS Form 7004.
IRS Form 7004: http://www.irs.gov/pub/irs-pdf/f7004.pdf
Intructions for IRS Form 7004: http://www.irs.gov/pub/irs-pdf/i7004.pdf
A corporation with any other fiscal year-end has a due date of: the 15th day of the third month following the close of the tax year.
The extention is only an extention of time to file, not an extention of time to pay any tax due.
Per the form’s instructions, the IRS will NOT send a comfirmation of approval; they will only notify you if the request is disallowed.