90 days before you check-in, and you can make them for up to 10 days after that date if you stay on-site. They'll ask for your resort confirmation number, which restaurant, day and time you want the ADR for and hopefully you'll get what you're looking for. Make sure you keep track of your ADR confirmation numbers too!
I'm not sure about off-site guests though... I think they just ask for name, but I'm not sure.