When to file. April 15 of each year is the due date for filing your federal individual income tax return if you are a calendar year filer whose tax year ends on December 31.Your return is considered filed timely if the envelope is properly addressed and postmarked no later than April 15.owever, an extension of time to file is not an extension of time to pay. You will owe interest on any past-due tax and you may be subject to a late-payment penalty if the payment of tax is not timely (by your due date).