Buying Your Tickets
Unlike, say, flight prices, ticket prices don’t vary by when you make the booking (they do vary by the date you visit, though). They also don’t sell out (except maybe two days a year, at the gate), like hotel space might. So you theoretically can wait until the day before your trip to buy your tickets.
You won’t do this though. Whether it’s FastPass+ reservations (discussed below) if those are available or Parks Pass Reservations (discussed above and below), you need tickets ASAP to start locking your trip down.