26 U.S.C. § 7703: Determination of marital status

the determination of whether an individual is married shall be made as of the close of his taxable year; except that if his spouse dies during his taxable year such determination shall be made as of the time of such death; and an individual legally separated from his spouse under a decree of divorce or of separate maintenance shall not be considered as married. an individual who is married (within the meaning of subsection (a)) and who files a separate return maintains as his home a household which constitutes for more than one-half of the taxable year the principal place of abode of a child (within the meaning of section 152(f)(1)) with respect to whom such individual is entitled to a deduction for the taxable year under section 151 (or would be so entitled but for section 152(e)), such individual furnishes over one-half of the cost of maintaining such household during the taxable year, and during the last 6 months of the taxable year, such individual’s spouse is not a member of such household,