According to the official Tokyo Disneyland website, they're closed until further notice, so there is no official date right now! Since Tokyo is currently in coronavirus lockdown, Disneyland probably won't reopen very soon. You can always check this list of closed facilities in Tokyo to see what's closed and what's open! They'll probably announce an opening date when the situation with COVID-19 calms down.
I started in my dream
That, I believe, will all depend on how the situations turn out. At the moment they have announced it to be Mid May, but originally they announced it to be 20th of April.
Thanks for the information!