Businesses organized as partnerships, including multi-member LLCs, and S-Corporations need to file Form 1065, or 1120S by March 15 if they are a calendar-year business. Businesses using a fiscal year need to file tax returns by the 15th day of the third month following the close of the tax year. For example, if a business uses a July 1 to June 31 tax year, the tax return would be due October 15.