Corp. and S Corp. Tax Due Event Date: Monday, 03/15/10  Corporations: File Form 1120 for 2009 calendar year and pay any tax due. For automatic 6-month extension file Form 7004 and deposit estimated tax. S Corporations: File Form 1120S for 2009 calendar year and pay any tax due. Furnish a copy of Sch. K-1 to each shareholder. File Form 2553 to elect S Corporation status beginning with calendar year 2010. Close Window