Disney World announced that guests looking to book vacations for 2021 can do so starting June 24th others are reporting that it is the 28th. I would play it safe and try the 24th, but I know that multiple sources are saying the 28th as well. You will be able to book hotel packages and theme park tickets for arrivals starting 2021. You will also be able to see park availability for the dates you are looking for to avoid any chance of a park not being open for the date you book (no park reservations have been sold out past opening week).
Tune in Wednesday night at 8PM est as this will also be discussed during our live taping of The Behind the Ears Podcast. If you are ready to book a trip we recommend that you contact Kingdom Destinations, they are our official TA for the podcast.
So if you book a park and that morning r night before you decide you want to go to another park and that park has availability can you switch your reservations to another park. Not just asking to do it when it opens but also for next year. Booking a year in advance what parks you want to go to for each day of your vacation.