Hey there, Henry! Welcome to planDisney. Thanks so much for stopping by with your question!
Henry, currently I do see May 21 available in the
reservation availability calendar, but reservations go fast, so I would make the change quickly if you can.
To change a reservation, first you must cancel the current reservation you are holding, then make a new reservation. To cancel a reservation, log in to your
Disneyland account, go to My Disneyland, Ticket and Reservation Details, View My Park Reservations, and there you’ll see the list of reservations you are holding. There is a link next to the reservation to cancel.
If for any reason you need to
upgrade your ticket to select your new date, upgrades are possible. You can
contact Ticket Services to upgrade and make a reservation for your new desired date.
I hope this is helpful. The magic awaits!
Kamini