you'd need to buy your AP at Guest Services or the ticket booths, I suspect the ticket booths/Guest Services office are only open from 8am, as thats when the security gates open for EMH. So you wouldn't get in for EMH at 8am, but after buying your AP at the gates of the park you could be in pretty soon after that

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You just have to remember to set aside some time that same day to go to the AP office inside the park with your passports and all of you who are getting APs, to get your AP made permanent.