Generally, an S corporation must file Form 1120S by the 15th day of the 3rd month after the end of its tax year. For calendar year corporations, the due date is March 15, 2017. A corporation that has dissolved must generally file by the 15th day of the 3rd month after the date it dissolved.
If the due date falls on a Saturday, Sunday, or legal holiday, the corporation can file on the next day that is not a Saturday, Sunday, or legal holiday.
If the S corporation election was terminated during the tax year and the corporation reverts to a C corporation, file Form 1120S for the S corporation's short year by the due date (including extensions) of the C corporation's short year return.
Extension of Time To File. File Form 7004 Application for Automatic Extension of Time To File Certain Business Income Tax, Information, and Other Returns, to request a 6-month extension of time to file. Generally file Form 7004 by the regular due date of the return.