Its 6:00AM to use the website/app to book ADRs or 7:00AM if you call the dining reservations line. I usually get on my computer at 6AM bc I think its faster, but 1 time the website was down or something so I had to wait until 7 to call Disney. I got everything I wanted but it took FOREVER! I was on the phone for an hour in a half trying to book all my ADRs...but on Disney's end I was on the deluxe dining plan and was booking 3 meals a day for like 8 days or something so it was a lot of booking. If you are only booking a few ADRs though I don't think it really matters whether you go online to book or call to book.