We don't know yet when Disney Cruise Line will set sail once again, but we know that when they do they will offer some flexibility in cruise dates for guests that have booked by August 31st, 2020. The new policy allows for guests to change their sail date up to 15 days in advance of sailing without penalty on sailings through March 2021.
See the details here:
https://www.disneyfoodblog.com/2020/06/24/news-disney-cruise-line-is-now-offering-cruise-date-flexibility/