For any single-day tickets, you will be required to designate a "start" date for them, which will be your designated date of use, and then must also pick the actual park you want (prices vary among the parks),
For multi-day tickets, you just need to designate a start date that you choose (such as day of arrival) and then can use them while there, e.g., I believe if you choose a six-day ticket and pick the arrival date as your start date, you will have a period eights days to actually use them.
As to making actual park reservations, you no longer need to reserve a park with multi-day tickets and can use the ticket daily for any park. If you have an annual pass, you will need to make park reservations in advance for each of the days you plan to be in a park. You should not wait too long to make such reservations because of the risk that a park could be deemed already full when you act to reserve, meaning, essentially, annual pass holders can no longer reserve the park, although any multi-day ticket purchasers can still use the park without any reservation)