I can confirm 8am EST / 5 am PST -- used it Tuesday to book our Christmas trip to DL.
At 4:59 am PST I had my selection made for resort & room type with the mouse hovering over the submit button. With iPhone in hand watching the clock, as it turned to 5:00am PST I clicked submit and was able to get my desired room. A few minutes later they were gone.
The confusing thing to me was that the box on the right of the booking page showed I was able to book my desired dates once midnight hit - you know the "you can book stays at your home resort begining ..." but I wasn't actually able to until 8:00am EST. I've checked and the same thing has happened every day since so I guess the website updates at midnight but the reservation system doesn't until 8am