Form 1041: Estates or trusts must file Form 1041 by the 15th day of the fourth month after the close of the trust's or estate’s tax year. For example, for a trust or estate with a tax year ending December 31, the due date is April 15 of the following year. A trust or estate with a tax year that ends June 30 must file by Oct. 15 of the same year.
Form 1041-A: Form 1041-A is a calendar year return which is due by April 15.