I called them directly at 9 am EST at my 180 day mark. Since I was staying on site, I gave them my resort reservation number. I was given a choice of two dates out of my week long stay. The first couple of dates were already taken, so we went with a date later in the trip. Try to be flexible with a date if you can.
It was a fantastic experience that we hope to do again one day.
Good luck trying to get a date. Always ask to be put on the wait list if you can't get the date you want.