Hey there Gerardo,
Thanks so much for stopping by planDisney with your question!
Based on when I'm answering your question right now, it looks like you're hoping to visit the Happiest Place on Earth tomorrow, August 31. Currently, to visit one of the parks of the Disneyland Resort, you need valid
theme park tickets and
theme park reservations for each day you're planning to visit.
With the theme park tickets, you can purchase those whenever and just need to use them by their expiration, which you can find in your confirmation email when you purchase them or at the bottom of the ticket purchase pages.
Theme park reservations are what can be subject to availability and that availability depends on the type of ticket you purchased.
In other words, theme park tickets in general will not sell out (unless they're part of a special event or promotion), but what could get booked up are the available theme park reservations - especially for busy days.
Having said that, I've checked the
theme park reservation availability calendar and it looks like, as of the time that I'm answering this question, both parks have availability for August 31! While you could purchase them in person when you arrive on-site (as long as there are available theme park reservations for that day), you can skip the line by purchasing them online and making your reservations in advance. That way you'll be ready to enter the parks when you arrive tomorrow.
If it were the case that there were not theme park reservations available for a given day then you would also not be able to purchase them in person either. However, again, it looks like there are currently reservations available for both parks as far out as December 28, so you should be good to go!
I hope you have a fantastic time visiting the Disneyland Resort, Gerardo. Thanks again for your question!
Have a Magical Day,
Greg