D.C. Mun. Regs. tit. 9, r. 9-106
All returns of income for the preceding taxable year required to be filed under the provisions of the Act shall be filed with the Deputy Chief Financial Officer on or before the 15th day of April of each year; Provided, that returns made on the basis of a fiscal year which is not the same as the calendar year shall be filed on or before the fifteenth (15th) day of the fourth (4th) month following the close of the fiscal year; and Provided, further, that returns required to be filed for the preceding year under the provisions of Title 7 of the Act shall be filed on or before the 15th day of March in each year, except that such returns, if made on the basis of a fiscal year, shall be filed on or before the 15th day of the 3rd month following the close of the fiscal year.
If the last date for the filing of a return falls on a Saturday, Sunday, or a legal holiday, the last date for filing the return shall be the first business day following that Saturday, Sunday, or legal holiday.
D.C. Mun. Regs. tit. 9, r. 9-106