Scheduled c section?

So this is my second c section.. Obviously scheduled.. Do they do it a week before due date, or two weeks? I have really big babies (9,8.. And 9,10)  in labeled high risk right now, and get pre eclampsia. So I know it could be way earlier. But my last two made it full term thank god. Just wondering if anyone knew the protocol with this. I plan on asking my doc of course. Just curious now.