When and how did pony boys parents die?

Ponyboy's parents died in a car accident some eight months before the novel begins.

The country would be like that, I thought dreamily. I would have a yeller cur dog, like I used to, and Sodapop could get Mickey Mouse back and ride in all the rodeos he wanted to, and Darry would lose that cold, hard look and be like he used to be, eight months ago, before Mom and Dad were killed.