According to the Chicago Tribune, divorce of Obama Sr. and Stanley Ann occurred in 1964. Where and when—presumably Honolulu but I have not yet found that in the record; nor I have seen the actual date in 1964.
We need to identify the court where the divorce occurred; and get a copy of the complaint. In the normal divorce proceeding, counsel pleads the location of the marriage—likely if he did so here, that sheds some light on the facts; maybe if he does not plead where they were married, that suggests a purpose in not doing so.
This April 9, 2008 Time Magazine story is specific as to the marriage date:
"On Feb. 2, 1961, several months after they met, Obama's parents got married in Maui, according to divorce records. It was a Thursday. At that point, Ann was three months pregnant with Barack Obama II. Friends did not learn of the wedding until afterward. "Nobody was invited," says Abercrombie. The motivations behind the marriage remain a mystery, even to Obama. "I never probed my mother about the details. Did they decide to get married because she was already pregnant? Or did he propose to her in the traditional, formal way?" Obama wonders. 'I suppose, had she not passed away, I would have asked more.'"
Note that the wedding date is taken from the divorce record; I've read elsewhere (but don't have the specific link) that the marriage certificate can't be found.
About the divorce:
"Ann filed for divorce in Honolulu in January 1964, citing "grievous mental suffering"—the reason given in most divorces at the time. Obama Sr. signed for the papers in Cambridge, Mass., and did not contest the divorce."