As soon as you get your KTTW card!!!
It will have a lot of info on it. Your name, your muster station, and...your dining rotation!
There will be a weird looking string of letters (ex., APLAAPL). Those are the dining rooms you'll be in for the cruise - A would be Animator's Palate, P for Parrot Cay, L for Lumiere's. Naturally, the letters will match the names of the MDRs on YOUR cruise (my example is from my NYE Magic Cruise!), but you should be able to figure it out pretty easily!
Have a great trip!