Currently, park tickets can be purchased in a couple of different ways. Most people have actually already bought park tickets specifying which days they plan to be in the parks. However, even those tickets have some wiggle room, so if you bought, for example, a three day ticket starting July 20, you actually have a couple extra days in which you can use all three days of that ticket. Now, it appears that guests will be restricted to specific dates. Alternatively, by paying a premium you could make your tickets usable essentially whenever you want.