Employers: For Social Security, Medicare and withheld income tax, file Form 941 for the third quarter of 2015. Deposit any undeposited tax. (If your tax liability is less than $2,500, you can pay it in full with a timely filed return.) If you deposited the tax for the quarter in full and on time, you have until November 10 to file the return. For federal unemployment tax, deposit the tax owed through September if more than $500.