Supreme Court Could Make History on Same-Sex Marriage, or Not WASHINGTON - It is usually impossible to say when the Supreme Court will announce any particular decision. There is one exception: On the last day of the term, when the court takes action on every remaining case, the process of elimination supplies the answer. The last day of the term is Wednesday. The court has yet to issue decisions in two momentous cases on same-sex marriage. Those facts in combination mean that shortly after 10 a.m. the justices will announce their rulings on challenges to two laws that define marriage to include only unions of a man and a woman. One case, from New York, concerns the federal Defense of Marriage Act of 1996, which denies federal benefits to gay and lesbian couples married in states that allow such unions. The other, from California, challenges Proposition 8, the state's ban on same-sex marriage. The rulings will come against the backdrop of a rapid shift in public attitudes about same-sex marriage, with recent polls indicating that a majority of Americans support allowing such unions. When the justices heard arguments in the two cases in March, nine states and the District of Columbia had laws allowing same-sex marriage. Since then, three more states have enacted such laws. The New York case, United States v. Windsor, No. 12-307, challenges the part of the 1996 law that defines marriage as between only a man and a woman for the purposes of more than 1,000 federal laws and programs. Another part of the law, not before the court, says that states need not recognize same-sex marriages from other states. The case concerns two New York City women, Edith Windsor and Thea Clara Spyer, who married in 2007 in Canada. Ms. Spyer died in 2009, and Ms. Windsor inherited her property. The 1996 law did not allow the Internal Revenue Service to treat Ms. Windsor as a surviving spouse, and she faced a tax bill of about $360,000 that would not have applied to a spouse in an opposite-sex marriage. Ms. Windsor sued, and last year the United States Court of Appeals for the Second Circuit, in New York, struck down the 1996 law. Should the justices strike down the law, married same-sex couples would start to receive federal benefits. Should they uphold the law, the current state of affairs for married same-sex couples - Justice Ruth Bader Ginsburg called it "skim milk marriage" when the case was argued in March - would continue. No ruling in the case on the 1996 law would require states without same-sex marriage to adopt it. The case is procedurally tangled. The Obama administration argued that the law is unconstitutional, though it continues to enforce it. House Republicans intervened to defend the law, though it is not clear that they were entitled to represent the interests of the United States. That leaves the possibility that no party before the Supreme Court had standing to challenge the appeals court's decision. The California case, Hollingsworth v. Perry, No. 12-144, was filed in 2009 by Theodore B. Olson and David Boies, two lawyers who were on opposite sides in the Supreme Court's decision in Bush v. Gore, which settled the 2000 presidential election. The suit, brought on behalf of two same-sex couples, argued that California voters had violated the federal Constitution the previous year when they overrode a decision of the state's Supreme Court allowing same-sex marriage. Lower federal courts agreed with Mr. Olson and Mr. Boies, striking down Proposition 8. The justices have several options in the California case. They could reverse the appeals court, leaving California's ban on same-sex marriage in place. They could affirm the appeals court's ruling on a theory that would allow same-sex marriage only in California. Or they could address the broader question of whether the Constitution requires states to allow such marriages. It is also possible that the court will give no answer on the merits, deciding instead that it was powerless to hear the case because no party before it was entitled to appeal from the decisions of the lower courts. (The California officials who lost in the lower courts declined to appeal. Supporters of the ballot initiative did appeal, but it is not clear that they were entitled to step into the government's shoes to do so.) That last option - dismissal on standing grounds, or something similar - would lead to short-term confusion, but many legal experts say they expect it would result in same-sex marriage returning to California in a matter of weeks.