You can use it ASAP
I just went last weekend and we bought our pass online and then they give you a pass card at the gate so you can use it ASAP.
If you buy a 2018 season pasa now, you can use it for the rest of 2017 and all of 2018.
I believe that you go to the park and pick up the yearly pass and you can use it as soon as you buy it.
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