Hotel
Room blocks for the wedding weekend have been set aside for guests. Please mention "Lilly & Jon Wedding" when booking your room.
Train
A great option for those traveling from NYC is to take the New Jersey Transit Northeast Corridor Line from Penn Station to Princeton Junction Station. From there you can Uber or take a Taxi to the hotel.
Flight
Princeton is about equidistant (~1 hour and 15 minutes) from both Philadelphia and Newark Airport. Traffic is a big determinant of the travel time. A closer, but more limited option would be Trenton Airport.