These conditions are to be applied with respect to the return for each of the taxable years of the decedent for which a joint return may be made if more than one such taxable year is involved. Thus, in the case of husband and wife on the calendar year basis, if the wife dies in February 1957, a joint return for the husband and wife for 1956 may be made if the conditions set forth in this subparagraph are satisfied with respect to such return. A joint return also may be made by the survivor for both himself and the deceased spouse for the calendar year 1957 if it is separately determined that the conditions set forth in this subparagraph are satisfied with respect to the return for such year. If, however, the deceased spouse should, prior to her death, make a return for 1956, the surviving spouse may not thereafter make a joint return for himself and the deceased spouse for 1956.
26 C.F.R. §1.6013-1