Hi Angela!
Welcome to planDisney! I'm so happy to be your
Disneyland Resort planEAR today!
This is a great question! Lately it does feel like a lot of our Disney days are very rigid, having to be decided on weeks or months in advance thanks to the new
Park Pass Reservation System in the addition to date-based theme park admission tickets. However, I'm very happy to say that there is still quite a bit of flexibility to be found at The Happiest Place on Earth!
I see that you already have your theme park tickets to
Disneyland Park. To be able to also visit
Disney's California Adventure Park also on the 21st, what you'll do is upgrade your current tickets to a Park Hopper, which will then give you admission benefits to both parks in one day. You might be able to make this upgrade now in the
Disneyland Mobile App. However, you can also do this in person at the ticket booths in The Esplanade upon your arrival, especially if you're wanting to pay for the upgrade using cash or a gift card.
With the Park Pass Reservation system, you should have already selected Disneyland Park as the park you wanted to visit that day. However, by upgrading to a Park Hopper you now have a choice to make: you can either start at Disney's California Adventure Park (at whatever time you arrive, you do
not need to be present at the park's opening time) and then you may enter Disneyland Park any time
after 1:00 P.M.; Or, you can flip that by starting in Disneyland and ending in California Adventure. I just checked the
Theme Park Reservation Availability Calendar and both options are currently open, but double check before you make your upgrades. (Pro Tip: When looking at this calendar, make sure to select the 'Park Hopper' bubble above the calendar.)
I hope that this helped to plan your upcoming Best! Day! Ever! Angela! And if you have any more questions about spending the day at Mickey's original house, We're All Ears!
~Kristen