If the trust is filing a separate return, the trust must file using a calendar year. The initial trust return year will begin with the date of death and continue through December 31st of that year.
With approval from the executors of the estate and the successor trustees of the trust, the trust can elect to have its income taxed to the estate during the election period. (See page 4 of the Form 1041 instructions "Special Rule for Revocable Trusts".) If the election is made all income and deductions of the trust are taxed to the estate. The estate does have the option to elect a fiscal year. To make this election the parties must use IRS Form 8855 (link below).