With the reopening of the parks Disney have put in a new system.
You have to reserve the dates you wish to go to the park.
You can only enter one park a day and you need a park reservation as well as a valid ticket to go.
Currently they are not selling any tickets for 2020 so you can only go if you are an annual pass holder, already have a valid ticket or are at a Disney Resort holiday.
More details on the disneyworld website.