We checked in at 1 pm, no problems quiet place.
I think it was 6 but I also think they leave your package at the office so you can check in late. They are also closed all day Saturday. But unlike a state park they have a free shuttle down into Freeport. (Except Saturdays)
I can tell you from my experience that check in time is well after 9 because the weekend we were there people came in at 10:30 and made a huge racket. This is why I prefer to go to a state campground where they actually enforce the rules.
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