This is probably about the closest to a parade preview we have seen in recent times. This ran from 11:30am to midnight.How does CM previews work for parades? Do they restrict it to only MK CMs and offer them one night to come and see the parade? For attractions I think they do previews for multiple days and basically have cast members reserve a time and day in advance for them and sometimes family members. They could do similar for Starlight if it will be offered to more than just MK cast.

Walt Disney World Cast Members will be the first to see the return of 'Happily Ever After' at Magic Kingdom with a special preview ahead of its April 3 official reopening
Walt Disney World Cast Members will be the first to see the return of 'Happily Ever After' at Magic Kingdom with a special preview ahead of its April 3 official reopening
