Howdy, Holly! Thanks for stopping by planDisney with your question!
Disneyland Resort theme park reservations can be made up to 120 days in advance. So that means that the calendar currently extends to mid-April 2022. If you are planning a trip for sometime early next year, I suggest purchasing your tickets and making your theme park reservations as soon as you are able. Reservations tend to fill up fast, especially during popular travel times like holidays and spring break.
Another thing to look into before purchasing your tickets is if you want to add on the
Disney Genie+ feature at the time of purchase. Disney Genie+ is a digital service in the Disneyland app that allows you to select the next available return time for select attractions and then skip most of the wait by using the Lightning Lane. The cost is $20 per day, per ticket. If you add this on when buying the tickets for your trip, it will apply for the duration of your tickets. If you wait to add it on later, you will have to purchase it daily once you are in the parks.
I hope this helps. Have a magical day!
-Linda