This is the last day to file tax returns.As most people know, the Canada Revenue Agency will assess a penalty if the return is not filed in time. A postal mark prior to midnight tonight will do. The failure to file penalty is separate a more serious than the interest that must be paid on unpaid taxes. The penalties start at five per cent of the amount owing, plus one per cent of the balance owing for each full month that the return is late — and compound daily interest is charged on the total amount due. If you file late more than once in a four-year period, the penalties can double.