If I remember correctly the events of the novel and movies start in late 1945 (with flashbacks obviously). By the time Michael makes his move against the other families, he's been married to Kay long enough that their son is already reading (remember he jokes with the Don about the funny papers); so it has to be at least the early 1950s. He kills the police captain in early 1946, hides out in Sicily, comes back, waits a year before he even contacts Kay--so they're not married until at least 1948.