Generally, a domestic partnership must file Form 1065 U.S. Return of Partnership Income by the 15th day of the third month following the date its tax year ended (as shown at the top of Form 1065).
To file for an extension, use Form 7004 Application for Automatic Extension of Time to File Certain Business Income Tax, Information, and Other Returns. Form 7004 can be filed electronically. For more information, see Instructions for Form 7004.
If the due date falls on a Saturday, Sunday, or legal holiday, file by the next day that is not a Saturday, Sunday, or legal holiday.