Many Disneyland fans this month have been going on a ride that few of them ever wanted. Long before anyone can get on Space Mountain or Haunted Mansion, Disneyland fans must buckle up for a trip through Disneyland's availability calendar.

December began with the entire month apparently "sold out," with reservations to visit Disneyland and Disney California Adventure unavailable to anyone - including daily ticket buyers as well as Magic Key annual passholders. But availability has opened and closed on various dates throughout the month, leading some fans to check Disneyland's app and website obsessively, looking to pounce on the first available dates to book with their tickets or passes.

Why is this happening? Let's dive into what factors drive availability for Disneyland park reservations.

Disneyland implements its park reservation system when it reopened last April. The initial idea was to limit the number of people visiting the park while it operated under state-mandated capacity restrictions. Those restrictions are gone now, but Disneyland has kept its reservation system as a way to control both the number of and type of ticket-holders visiting its theme parks.

The Disneyland Resort maintains separate availability calendars for its Magic Key holders, Park Hopper ticket holders and one park per day ticket holders. That suggests that Disneyland is limiting the number of each type of ticket holder being admitted to the park on any given date, beyond limiting the total number of visitors. The effect seems to have been to reduce the number of passholders in the park in favor of creating space for more single and multi-day ticket buyers. In fact, Disneyland is getting sued now over that.

But how is Disneyland deciding how many of each ticket class to allow into the parks? And how is that affecting the ever-changing availability that fans are seeing on the calendars?

Disneyland has to start with its overall park capacities, and those can vary by day, depending upon park operating hours and what attractions, restaurants, and facilities are - or are not - open. If a major attraction suffers an unexpected extended downtime, that's going to reduce capacity and force Disneyland to cut availability. On the flip side, if the resort decides to extend park hours, or a big ride comes back from refurb, that would expand availability.

Disney also needs to consider when people are likely to enter the park and how long they are likely to stay when estimating how many reservations spots it can make available on any given dates. And that's the thing. Disneyland is estimating availability on every date of that calendar. Like a restaurant manager trying to guess how long parties will stay at their tables, Disneyland has to use its experience to estimate how many people will leave the park during the day, creating space for new visitors.

Park hopping also complicates that equation. That's part of the reason why Disney chose to prohibit park hopping in the mornings, as it's trying to make its crowd forecasting less complicated. That's also part of the reason why Park Hopper ticket holders pull from a different reservation pool than other types of Disneyland ticket holders.

Perhaps the biggest trick Disneyland must pull off with its reservation system is properly balancing those reservation pools. How many slots does each type of ticket holder get each day? Disneyland wants to maximize its daily ticket revenue, so it must start by estimating how many single and multi-day tickets it can sell for use on any given date. It can then assign any remaining capacity to Magic Key holders.

Yet Disneyland can't just use Magic Key holders as filler. Given the number of Magic Keys the resort has sold, it needs to ensure enough availability to allow those passholders to be able to make their allotted number of reservations within a given month. Otherwise, Disneyland could be facing an even more enormous and unwanted backlash, not to mention more lawsuits. Disneyland's decision to stop selling certain types of Magic Key passes reduces some of that pressure, as Disneyland no longer needs to account for future growth in the number of those Magic Key holders when estimating the availability it must reserve for passholders.

But the whole availability forecast starts with projected daily ticket sales. Disneyland has decades of sales data to guide that estimate, but the past several months have scrambled business forecasts throughout the economy, especially in the travel and entertainment businesses. Since the ultimate goal of a reservation system is to prevent having to turn away guests at the gate, it's understandable that Disneyland would rather start low with the number of reservations available, especially for Magic Key holders. If daily ticket sales do not meet forecasts, then Disneyland can add Magic Key availability.

Cancellations also open available spots. Since Disneyland can restrict reservations for frequent no-shows, most Magic Key holders try to cancel if they decide not to use their reservation on a given date. Since Southern Californians also traditionally hate going out in the rain, a wet forecast can open up availability, as we have seen this week and next. Really want to see Disneyland for the holidays this year, even in the rain? Check availability Wednesday night, because if Thursday's forecast holds, a bunch of passholders will be canceling.

So should Disneyland just ditch its reservation system and go back to its first-come, first-served free for all? I hope not. Disneyland was just too crowded too often before the pandemic closed the parks. Even as crowded as the parks have felt at times in the past month, they have not approached that pre-pandemic level of discomfort. If Disneyland makes a change in its ticketing system, I would rather see it do away with Magic Key passes than drop reservations. (I think that might actually lead to Disneyland eventually dropping prices on some regular tickets, as it looks to fill the parks in the absence of annual passholders.)

As time passes, Disneyland will gather more data and build more experience forecasting availability. So the roller coaster thrill ride we saw with December availability this year might smooth out to a more pleasant experience.