What are some of the best astronomy gatherings in the USA?
NEAF is pretty big (and probably the biggest).  There's also Southern California Astronomy Expo (SCAE).  

For observing, there is the Texas Star Party in west Texas.  There is the Winter Star Party in the Florida Keys (that site for that event was severely damaged by a hurricane a couple of years ago, had to relocate up-state, but are now back in the keys.  This past year the had to limit attendees because the camp wasn't fully restored.

A little closer to home for you (in Pennsylvania)... is the Cherry Springs Star Party (there is also the Black Forest Star Party at the same park ... different months).