Yes, it is very common for Disneyland park reservations to "open up" the day before or even the morning of a visit, even if the calendar previously showed "sold out." This happens because of Disney's cancellation policy: guests with "Tier" tickets or Magic Key passes must cancel their reservations by 11:59 PM the night before to avoid a "no-show" penalty or to remain eligible for a refund/rebooking. Consequently, there is often a wave of availability that appears on the reservation calendar late at night or early in the morning as people change their plans. For those desperately seeking a last-minute spot, the best strategy is to refresh the availability calendar repeatedly between 8:00 PM and midnight the night before you want to go. Additionally, Disney occasionally releases extra capacity for specific ticket types (like park hoppers) if they see that staffing and crowd flow allow for more guests. However, there is no guarantee, so relying on this method for a major holiday or a new ride opening is risky.