Most trusts have to use a calendar year. This from the Form 1041 instructions:
"Generally, a trust must adopt a calendar year. The following trusts are exempt from this requirement:
A trust that is exempt from tax under section 501(a);
A charitable trust described in section 4947(a)(1); and
A trust that is treated as wholly owned by a grantor under the rules of sections 671 through 679."
Note that the extension due date is only 5 1/2 months. Also, the letters assigning EINs to estates and trusts are pre-programmed to use a calendar year. Almost all the estates I do elect a fiscal year, so you can safely ignore the letter. Gail is correct that the reporting period is established when the first 1041 is filed, letter instructions notwithstanding. However, this only applies to estates and exempt trusts.