The estate tax return required by section 6018 must be filed on or before the due date. The due date is the date on or before which the return is required to be filed in accordance with the provisions of section 6075(a) or the last day of the period covered by an extension of time as provided in § 20.6081-1. The due date, for a decedent dying after December 31, 1970, is, unless an extension of time for filing has been obtained, the day of the ninth calendar month after the decedent's death numerically corresponding to the day of the calendar month on which death occurred. However, if there is no numerically corresponding day in the ninth month, the last day of the ninth month is the due date. For example, if the decedent dies on July 31, 2000, the estate tax return and tax payment must be made on or before April 30, 2001. When the due date falls on Saturday, Sunday, or a legal holiday, the due date for filing the return is the next succeeding day that is not Saturday, Sunday, or a legal holiday. For the definition of a legal holiday, see section 7503 and § 301.7503-1 of this chapter. As to additions to the tax in the case of failure to file the return or pay the tax within the prescribed time, see section 6651 and § 301.6651-1 of this chapter. For rules with respect to the right to elect to have the property valued as of a date or dates subsequent to the decedent's death, see section 2032 and § 20.2032-1, and section 7502 and § 301.7502-1 of this chapter. This section applies to estates of decedents dying after August 16, 1954.