If the written statement of election is made in a letter, it shall be signed by the taxpayer making the election if an individual or, if the taxpayer is not an individual, the letter must be executed in the same manner as required in the case of the income tax return of such taxpayer. The date of the making of the election shall be the date the return, amended return, claim for refund or credit, or letter is filed in the office of the district director, or the date the petition or amended petition is filed with The Tax Court of the United States. In case the election is made in a return filed before the last day prescribed by law for the filing thereof (including any extension of time for such filing), such election shall not be considered made until such last day. See section 7502 and the regulations thereunder with respect to the timeliness of filing an election where filing is done by mail and section 7503 and the regulations thereunder with respect to the timeliness of filing where the last day for filing falls on a Saturday, Sunday, or legal holiday.
26 C.F.R. §1.1335-1