Below is what the Disney site states about Dining.
https://disneyworld.disney.go.com/faq/dining-reservations/advance-reservation-restaurants/
- The Disney Reservation Center is available by phone from 7:00 AM to 11:00 PM Monday through Friday, and 7:00 AM to 8:00 PM on Saturday and Sunday.
- Reservation availability for select Walt Disney World restaurants typically begins each day at approximately 5:45 AM.
- Please be aware that if you cancel a restaurant reservation, your previously scheduled reservation time will immediately be made available to another Guest.
- All newly introduced dining events—such as special dining experiences and dessert parties—can typically be booked beginning at 7:00 AM on the day their offers become available.
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I have not had good luck with booking dining on-line before 7AM, Eastern Time.
I've had it "click in" at 7AM, but you can give it a shot.
Apparently, it works better with the
MDE APP rather than using the website.
Here's a very recent thread about Dining booking-
https://www.disboards.com/threads/adrs-at-6am.3631376/