Important Tax Return Due Dates For Calendar Year Entities:
Form 1040 Individuals: April 15, October 15 with 6 month extension on Form 4846
Form 1120 C Corporations: April 15, October 15 with 6 month extension on Form 7004
Form 1065 Partnerships: March 15, September 15 with 6 month extension on Form 7004
Form 1120S S Corporations: March 15, September 15 with 6 month extension on Form 7004
Whenever possible, you should file by the due date. If you're late, you may have to pay a penalty as well as interest on any over due taxes.
REMEMBER: An extension to file is NOT an extension to pay any tax due by the due date.