Treasury Regulations § 1.443-1

(a) Returns for short period. A return for a short period, that is, for a taxable year consisting of a period of less than 12 months, shall be made under any of the following circumstances: . . .

(2) Taxpayer not in existence for entire taxable year. If a taxpayer is not in existence for the entire taxable year, a return is required for the short period during which the taxpayer was in existence.. . .Although the return of a decedent is a return for the short period beginning with the first day of his last taxable year and ending with the date of his death, the filing of a return and the payment of tax for a decedent may be made as though the decedent had lived throughout his last taxable year.

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IRS used to accept returns filed before the end of the year for a decedent's final return.  I think they still do, but it confuses them.  You have to use the previous year's forms.  If someone dies in March and the survivors don't want to wait a year to settle the estate, it's an option, especially if there is no tax owed or a small balance due.