October 15 - Electing Large Partnerships File a 2012 calendar year return (Form 1065-B). This due date applies only if you were given an additional 6-month extension. March 15 was the due date for furnishing Schedules K-1 or substitute Schedule K-1 to the partners. October 15 - Social Security, Medicare and withheld income tax If the monthly deposit rule applies, deposit … [Read more...] about October 2013 Business Due Dates