You can! (It's actually a covered concrete slab- pavilion) If there's not too many people it shouldn't be a problem ** Best thing to do is call the Kinnelon Parks and Rec department. I would say under 20pl wouldn't be a problem. But Definitely call to be certain. And you'll need to be responsible for garbage/trash.
To add to Jen's excellent reply, most people have the party under the Pavillion, however I have seen tents put up as well either next to the Pavillion or in another part of the park nearby. It's a lovely location with short and long hiking options. Most people at the park don't seem to know about the white and yellow trails. Worth a visit (maybe not during a typical birthday, granted ). The white trail takes an hour if you hustle (including some running) or about 90 mins at a chill pace.
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