This depends on the type of return. Forms 706 and 706-NA: The due date for Form 706 and Form 706-NA is 9 months after the date of the decedent's death. If there is no numerically corresponding date in the 9th month, the due date is the last date of the 9th month. Form 706-A: Form 706-A is due 6 months after the taxable disposition or cessation of qualified use. Form 706-QDT: Form 706-QDT is due: On or after January 1 but not later than April 15 of the year following any calendar year in which a taxable event occurred, or a distribution was made on account of hardship; or nine months after: (a) the death of the surviving spouse, or (b) the failure of the trust to qualify as a Qualified Domestic Trust (QDOT).