Here's an important question: what was the purpose of all the premonitions if the girls end up being wrong? Well first of all, they are only wrong in a few ways: they were correct about serious earthquakes, since the next few weeks after their prophecy saw many serious earthquakes, just not exactly where or when they thought they would happen. Secondly, the date itself does end up being the date for the novel's major climax, the irresponsible affair between Kate and Hank.
Now, marriage is incredibly complicated, and marital affairs are an unfortunate part of life, but for Kate, the date is highly tragic for two reasons, the first being that she judged her sister and became embarrassed by her sexuality when she found out Violet was bi-sexual. But, Violet's relationship with Stephanie is not harming anyone, as Kate's relationship to Hank does. Secondly, Kate experienced the same premonitions for a fateful October 16, but instead of considering a wider point of view, she considered that the premonition only applied to literal earthquakes. These are both kinds of hubris, preventing Kate from seeing some part of herself that makes her uncomfortable, her dark side, one might say.
Look at Kate's name for a final piece of evidence. Her real name is Daisy, a flower like her sister's name, Violet. But interestingly, Daisy decides to go by Kate. We know from her backstory that she was often teased by kids who didn't understand her, so maybe that has something to do with this name change, but in any case, the point is clear: Violet's prophecy was not about earthquakes, it was about Kate's progress in her fate, probably toward divorce.
