J.H.
Definitely talk about the trip and get them excited about airplanes... But... I wouldn't start talking about it too early. Kids have a difficult time gaging "time." I would begin getting them excited about the trip one week before. This will keep them excited without having so much time pass that they begin to get nervous. They won't panic though unless you somehow let them know that you are nervous. Be confident and relaxed, and they will be too!