When and how did Pony’s parents die?

Ponyboy's parents died in a car wreck eight months prior.

Since Mom and Dad were killed in an auto wreck, the three of us get to stay together only as long as we behave.

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.