As all others have said, depends on the school. Try checking their websites to see if they have past years' dates uploaded so you can get an estimate. I had my interview mid-September and have been accepted (yay!), but this is also pretty early. I think most begin happening in October, with early October reserved for early decision candidates. Then, interviews continue every few weeks for the rest of the cycle.
Alternatively, if nothing is posted on the websites, try calling the admissions office. They should have an idea of when they're planning to interview candidates this cycle. It can be anonymous; they won't ask you for your name when you ask such a general question.